Bowser's cool, but I'm not sure if I'd want to play as him. Didn't play as him in Smash Bros. Melee/Brawl because of his lack of speed, keep thinking his size and slowness will put him at a disadvantage. On the other hand, he's very strong, so that worked out in SMRPG when a lack of speed wasn't as much of an issue (or even that noticeable). As for Mario Kart 64, I've never played as a heavyweight, so Bowser's out. ...heavyweight... why don't we have a Mario boxing game yet?
It was probably that darn Super Mario World Western Show video, but Mario and Bowser seem like a classic pair when fighting eachother (be it a life or death situation or just fighting over who gets Peach). Just seems more right for them to be villains. And it's that situation where you don't really want to see either of the two die permanently. While having Bowser at my side is great, maybe that's been diluted by how he's been goaded into being your ally in other games, including the Mario sports games.
But Bowser starring in his own game, sure, why not. There was a game featuring Peach, which blows my mind, why not Bowser.