Just how pervasive is this trope? Well, recently on Homestar Runner, Strong Bad got an email asking what he would do if he had the ability to transform. StrongBad immediately assumes that he'd have to somehow be Blessed With Suck:
"If comic books, cartoons, and Sci-Fi Original Movies have taught me anything, it's that shapeshifting comes with a bunch of boring rules and restrictions that limit its potential Turn-Into-A-Bulldozer-Whenever-I-Wantity. You can turn into a machine gun but not bullets, contemporary jazz turns you back to normal, you can only turn into presents your grandma's knitted for you. Crap like that. For example, let's say I could turn into any species... OF BALLOON ANIMAL!!??"
Moist from Dr. Horrible is an example of the first variation of this. His 'power" is that he is constantly damp. All he's really useful for is...well...making things soggy. In his own words: "At my most badass I make people feel like they wanna take a shower."