Growing fingernails is pretty useless on it's own... Though if they could be launched like projectiles that could be more useful. (Especially if they could put poisons on the tips...)
I would say that the ability to inflate like a balloon would be pretty useless in most cases. (Bouncing Boy from Legion of Superheroes being one example of "inflatable hero".) Sure he can become rubbery and inflate, but most villains either have pointy objects, access to pointy objects or sufficient powers to completely negate Bouncing Boy's inflation. So stab Bouncing Boy with a pin and he deflates and likely can't re-inflate for a while. Besides if they're a super villain they probably are powerful enough that getting hit with a ball, even one moving at high speeds isn't going to even budge them. (EX- if Superman can haymaker an enemy and they only go flying 10 feet without much visible damage, being hit by a ball is only a nuisance.)
There are probably more useless powers, but I can't think of them right now...