I think of 20% time as error correction for management. Engineers can route 20% of their time to what they think is important, and not what management thinks.
Nobody from management told me to rewrite the primitive array handling in the proto libraries for Java. It just irritated me that they didn't do what I thought they should, so I spent some time fixing it.
In my experience, 20% time is something that has management blessing. Otherwise it's just a risky side project where you're still accountable for 100% on your main assignment.
