Some games just should not be played with a mouse. I guess that makes them not so casual, but that's life. Sidescrollers are akward with mice, pacman games are weird, obviously FSP and some RPGs and racing games, just not designed for a mouse. I guess a mouse controll can be added for the ultra freaks who refuse to use a keyboard, but some games are just different. I still think games like sidescrollers and pacman are sort of casual, not for everyone, as they require skills to play and not just click around everywhere. Some mouse games arent as casual at all, take Diablo for example. That game is a click-fest, no keyboard needed to play, but it definitely isn't a casual game.
I think if the player likes the game, keyboard controlls arent going to stop them.