Bearings vs. bushings in rear derailleur jockey wheels - which should be more resistant to contamination?
I've replaced bushings with bearings in derailleurs I've had and noticed  improvement. I've had bushings get gunked up and bind or freeze, and have had bearings do the same, but over all when my derailleur is clean, there is less friction and the benefit is noticeable.

Derailleur pulleys, like all other parts of the drive train should be kept clean. I do most of my riding on trails, so gunk is an issue. I occasionally pop the cage off and wipe everything down, spritz with some citrus degreaser, and reassemble, about every fourth or fifth time I pull my chain off to clean it. 

I think bushings are cheap and useless. It makes absolutely no sense why any company that manufactures bicycle components would sell me a drive train component with bushings rather than a sealed cartridge bearing.

This is a fun question. I've thought about this quite a bit. This one area that I base my understanding completely on my own experience.