I know I've said this before, but among all the other reasons to hate the blue shell is this one: it is the only item that is no help whatsoever to the person who uses it. (With a few exceptions, which I'll get to in a minute.)
If you're in 8th place and you throw a blue shell, what the fuck good does it do you? It just rearranges the 7 guys who are in front of you. If you're that damn far back, taking out the guy in first place is going to do fuck-all to remedy the situation.
If I'm in last place I never hope for a blue shell, I hope for a damn bullet. Or, failing that, three red shells, or a star, or maybe mushrooms if I'm on a course with straightaways. (Of course, the formula that gives you more mushrooms when you're in last place does not account for course layout, and getting a bunch of mushrooms on Rainbow Road is a pretty big Fuck You. Well, maybe not in 7 where Rainbow Road actually has a pretty good number of places to use mushrooms, but in many of the other versions of Rainbow Road over the years.)
I also don't care much for the Bob-Omb. It's no use for offense because if you throw it in front of you you're pretty likely to crash right into it yourself, and it's not very good for defense because you can't use it to prevent items from hitting you, and you wouldn't want to use it on someone who's right up behind you because you'd get caught in the explosion yourself.
Lucky 7 is a generally pretty cool item that's largely marred by the presence of the Bob-Omb, because if someone gets too close to you they can explode you.
But yeah, the Blue Shell sucks. It sucks if you're in first place and it sucks if you're in last place. It's a terrible item.
BUT, as promised, there are the following exceptions:
1. If you actually manage to aim it so that it whacks someone right in front of you on its way to the first place guy. But a green shell would work just as well for this and a red shell would be better.
2. In games where you can play a race where there are only four racers. 8 racers is too damn many for the guy in last to make his way up to first just because the guy in first got hit, but if there are only 4 racers it's possible, especially if the first two or three are clustered close together.
3. If you're playing local multiplayer and trying to unlock shit. If you're in last place and throw a blue shell, it won't help you get up to first, but it might help the other human player get into first and then you both benefit from a greater likelihood that one of you will get a gold and unlock whatever it is you're trying to unlock. (Does not apply to Mario Kart Wii because you cannot unlock things in multiplayer on Mario Kart Wii. Fuck Mario Kart Wii.)
4. If you're doing GP and you've generally performed well but are doing badly in just this one race, disrupting the guy in first might mean one of the other bots finishes first and doesn't affect your lead in the rankings. (Like, say, you've gotten first place in two races with Bowser in second in both cases, but now you're on the third race and fucked up somewhere and are running pretty far behind -- there's an advantage to taking Bowser out so that someone else takes first so that it's harder for Bowser to overtake your overall lead.)