I think that would make it MUCH easier, so nah
.... You're kidding right?
It's not about Ease, it's about gameplay and convenience.
Can you imagine playing any other type of game that requires shooting. Placing down the controller, and reaching for an entirely separate button somewhere else, just to shoot a target? In a fast paced game, any sort, You'd miss 100% of the time.
If you want more difficulty, that's why there are difficulty options, such as Impossible mode. More bullets, a faster moving target. Possibly extra abilities not seen in lower difficulties. You don't want a game that's difficult to control and play.