It's the worst kind of game design, rewarding players for purposefully wasting their time in a GAME doing something un-fun. The only way it could be even slightly worth it is if it forces them to go off the beaten path, and finds something amazing for their trouble like a whole new level or boss, but that's unlikely. Usually, yeah, they're just stupid scavenger hunts or grinding competitive idiocy.
I am so very, very pissed that I will forever have 99% of the Emblems in Sonic Adventure 2, but the last 4 or so are for getting all A-ranks in the ones I've already completed... another reason to hate these hunts silly!