I haven't used Pleco that much, so I wonder what you're referring to. Is that:
1. The dictionary app (excluding the dictionary).
2. The built-in (free) dictionaries (PLC and CC-CEDICT).
3. The paid add-on dictionaries.
4. The add-on marketplace.
I think if you want to be fair, you shouldn't compare Pleco to apps based on EDict/JMDict, but rather to apps that can read EPWING dictionaries.
From what I can see and judge:
1. The dictionary functionality in the app doesn't seem any better than EBWin/EBMac/EBPocket [1]. In fact, the EB apps seem to offer more advanced functionality like dictionary groups and different search types (prefix/suffix/infix/exact/full-text).
On the other hand, Pleco is also more user friendly than EBPocket. Replacing dictionary groups by a Chinese-English/English-Chinese is a usability win for 99% of the users.
The Japanese-only documentation and retro homepage probably also deter users from using EBPocket. This is a shame, since the apps are pretty great (they do have English UI). If you just want a dictionary they are miles better than Imiwa/Yomiwa and all the other apps which are based on JMDict.
2. The built-in PLC dictionary seems better than JMDict. It's got better and clearer tagging for the headword and definitions and in-line examples (which you can only get by linking JMDict to another dictionary). These are not issues with JMDict itself though, since it seems to contain all this data (there is even a version that incorporates example sentences from the Tanaka corpus [2]). But most of the JMDict dictionary apps I've seen are not good at displaying it. The EPWING conversions I've found are also not great.
While the PLC dict is great, CC-EDict seems much worse than JMDict.
3. The quality of commercially available Chinese<->English and Japanese<->English dictionaries really depends on the publishers. Compiling and editing good dictionaries take years, and as far as I know Japan has been doing digital dictionaries for a pretty long time. In my eyes, the best general purpose J<->E dictionary (Kenkyusha [2]) is still better than the best commercial C<->E dictionaries, but it is aimed at Japanese speakers studying English and a bit hard to use for beginner Japanese learners.
4. I think the easy to use marketplace is where Pleco really shines. With Japanese dictionaries, you either need to buy the EPWING dictionary in a CDROM (I think it's out of print nowadays) or use the official app, which is pretty limited.
This is the real issue here. I don't know if it's Japanese publishers who don't want to sell the dictionary data to third parties in a marketplace model or the app developers themselves being content with JMDict. It's quite sad, since there used to be a vibrant ecosystem of add-on dictionaries on SD cards for physical electronic dictionaries.
Been thinking about adding EPWING dictionary support to Nihongo for a long time, maybe it’s time to make that happen. It seems to be the only way to get good J-J definitions in particular.
Mostly I mean the paid add-on dictionaries. In dictionaries, you're getting what you pay for.
The interface to the dictionaries is good, but very straightforward. I don't think there's anything magical about it.
> In fact, the EB apps seem to offer more advanced functionality like dictionary groups and different search types (prefix/suffix/infix/exact/full-text).
Those search types are also available in Pleco, but you have to read the documentation to learn about them. The app has no way of teaching you to use them.
> In my eyes, the best general purpose J<->E dictionary (Kenkyusha [2]) is still better than the best commercial C<->E dictionaries, but it is aimed at Japanese speakers studying English and a bit hard to use for beginner Japanese learners.
I think this is a real issue. The native language of the intended audience has a huge effect on how useful the dictionary is for whom. This really shows up in the E -> C dictionaries available for Pleco, which seem to be targeted at Chinese speakers even when they are paired with C -> E dictionaries intended for English speakers. ABC / ABE is an offender there.
But by way of an example of what having a good dictionary means, here are the CC-CEDICT and ABC entries for 以:
-----
1. to use
2. by means of
3. according to
4. in order to
5. because of
6. at (a certain date or place)
-----
"coverb" [preposition]
1. using; taking - 以合成橡胶代替天然橡胶 use synthetic rubber in place of natural rubber
2. because of - 以盛产红茶著名 famous for abundantly producing black tea
3. in order to; so as to - 以应急需 in order to meet an urgent need
suffix
（"empty" verb suffix) - 加以 add; 给以 give
fixed construction
1. 以 A 为 B: take/regard A as B
2. 以 A 来说: as far as A is concerned
bound form [meanings that the character may express when it is part of another word, but not when it is independent]
from a point on in 以下，以北 [etc.]
-----
In my opinion, if you're using the PLC dictionary, you're not using Pleco correctly. (CC-CEDICT has lower quality entries, but is nevertheless better than PLC because it has entries that can't be found in other dictionaries, such as the Chinese names of foreign celebrities. PLC has OK quality, much better than CC-CEDICT, but if you're looking for entry quality you should use the paid dictionaries, which are quite far ahead of PLC.)
1. The dictionary app (excluding the dictionary).
2. The built-in (free) dictionaries (PLC and CC-CEDICT).
3. The paid add-on dictionaries.
4. The add-on marketplace.
I think if you want to be fair, you shouldn't compare Pleco to apps based on EDict/JMDict, but rather to apps that can read EPWING dictionaries.
From what I can see and judge:
1. The dictionary functionality in the app doesn't seem any better than EBWin/EBMac/EBPocket [1]. In fact, the EB apps seem to offer more advanced functionality like dictionary groups and different search types (prefix/suffix/infix/exact/full-text).
On the other hand, Pleco is also more user friendly than EBPocket. Replacing dictionary groups by a Chinese-English/English-Chinese is a usability win for 99% of the users.
The Japanese-only documentation and retro homepage probably also deter users from using EBPocket. This is a shame, since the apps are pretty great (they do have English UI). If you just want a dictionary they are miles better than Imiwa/Yomiwa and all the other apps which are based on JMDict.
2. The built-in PLC dictionary seems better than JMDict. It's got better and clearer tagging for the headword and definitions and in-line examples (which you can only get by linking JMDict to another dictionary). These are not issues with JMDict itself though, since it seems to contain all this data (there is even a version that incorporates example sentences from the Tanaka corpus [2]). But most of the JMDict dictionary apps I've seen are not good at displaying it. The EPWING conversions I've found are also not great.
While the PLC dict is great, CC-EDict seems much worse than JMDict.
3. The quality of commercially available Chinese<->English and Japanese<->English dictionaries really depends on the publishers. Compiling and editing good dictionaries take years, and as far as I know Japan has been doing digital dictionaries for a pretty long time. In my eyes, the best general purpose J<->E dictionary (Kenkyusha [2]) is still better than the best commercial C<->E dictionaries, but it is aimed at Japanese speakers studying English and a bit hard to use for beginner Japanese learners.
4. I think the easy to use marketplace is where Pleco really shines. With Japanese dictionaries, you either need to buy the EPWING dictionary in a CDROM (I think it's out of print nowadays) or use the official app, which is pretty limited.
This is the real issue here. I don't know if it's Japanese publishers who don't want to sell the dictionary data to third parties in a marketplace model or the app developers themselves being content with JMDict. It's quite sad, since there used to be a vibrant ecosystem of add-on dictionaries on SD cards for physical electronic dictionaries.
[1]: http://ebstudio.info/manual/EBPocket_iPhone/ [2]: Specifically 研究者新英和大辞典 and 研究者新和英大辞典