The thing with round rect buttons is not that they're irreconcilably ugly, but that they're only meant to be used in pinstriped table views. It's rare to see them used correctly anymore since pinstripes are dead as a popular design style in third-party apps. (The original Tweetie 2 was a notable exception.)
Instead they just get used because they're the only freely placable styled button in Interface Builder.
Well like it or not with the App Store you need to go Big or go Home. There is almost no place for default interface elements in most Apps because you get lost in a sea of crappy generic apps. I only use them as a placeholder to wireup the button in IB and the off to custom button land I go.
I like how it's vector, that helps keep bundles small (instead of a bunch of custom button PNGs I usually have to have in there)
I had exactly this idea for a while I had not gotten around to implementing it, my plan was to make it open source.