My main beef with Blue Shells is that they exist they can't be countered. By anything. Essentially, if you're in First, you have no way of dodging the thing except giving up your placing in the race/.
Also, chances are, if one is placing badly enough to have a Blue Shell in the first place, he's not going to gain anything from taking out the player in First unless he's a sadist. Or has a grudge. Basically, the game punishes someone for having enough skill to get in First place (of course, with Bullet Bills and Mega Mushrooms, it doesn't take much skill to do that, either).
[/rant]