1. Spike is not useless. I think that's plain to see, I don't need to say more.
2. Spike and Rarity will never actually become a thing. The age difference is of course a big reason, but it would just be awkward all around and weird. If the show was going to make it canon I think they would of already anyways.
I see nothing wrong with a Dragon and Pony relationship, however, Spike and Rarity just shouldn't, and I assume won't, be a thing.