Originally we planned on doing different types of guns that you could switch between, but unfortunately we didn't have time to implement that. As for why we have the player hold down the mouse button, it's because 1.) not all of the planned weapons would be fully automatic and 2.) there is some subtle knockback to the player when shooting, and we figured players wouldn't want to be constantly being knocked back with no way to stop it.
Thank you for your feedback and ideas!