I wouldn't trade my nonperfects if they've been powered up beyond 30, and I would trade nearly anything for perfect legendaries.
I wouldn't trade for unevolved nonperfects since the moveset is undetermined.
I would only trade other bad perfects for bad perfects, decent perfects (like Donphan) are interesting though, and I would trade for them. I would probably use it over a 5th Machamp in a TTar raid.
This is all because decent or good perfects I will consider maxing out someday since there is no better alternative in terms of that species.