There are a few reasons in which I believe that Spike and Rarity will never get along. Firstly, and I'm shocked no-one else commented on it, is that fact that dragons are prone to greed in the series, and Rarity is the element of generosity. It's like two sides of a coin-you need both for the coin to work, but they can never meet.
Secondly, it does still have to stay under the 'kids-show' aspect, and having a relationship in a show may not be the best thing to show little kids that watch the show because they are kids. While I personally think it doesn't matter, I have other brony friends that think along those lines.
Thirdly, dragon/pony relationship just won't work. I mean, I know it would be an interesting match up, but it's like having a relationship with your dog. Mind you, that hasn't stopped people in this world from doing that anyway.
Fourthly, age difference. If Rarity does try to hook up with Spike, one word is enough to describe her-PAEDOPHILE.
I would personally like to see it work, but life's a bitch, what can you do?