I've restarted my emerald game at least three times over, I've restarted leaf green once or twice, and I've restarted both Gale of Darkness and Colosseum once each. Mostly, it was because I had really enjoyed the games and wanted to play through the story mode again, trying out different teams that I hadn't tried before.
I've never restarted any of my gen IV games though, and I doubt I ever will. I'll certainly never restart HeartGold or Platinum. The only reason I restarted my Gen III games was because I was able to gut them of the rare pokemon and items and store them on another game. Or in the case of XD and Colosseum, I just needed a different memory card to start a new game. If I had an efficient way to move my pokemon from Pearl, I'd think about restarting that one too. But, alas, I have only the one DS, so transferring pokemon and items is a royal pain.
