Direct tower attack decks did have a heyday, but they're a lot harder to pull off after other defensive measures were added. Nowadays, it's rare to see such a strategy except in short-mode games.
I agree that raising the cap to 75 in long mode probably wouldn't be used to counter tower attacks as often as it would be used as an early boon in tower build decks. (At least, I know I'd be abusing it that way. =D ) The cap at 50 seems appropriate to me.