I think limiting multi classing stops the players from be able to play certain concepts and that is not a good thing. Instead if you are worried that it might be abused make a rule that players must talk to you a head of time and show through game play why it makes sense for the character to multi class.
To often multi-classing is, in my opinion, unnecessary hoop jumping for viable starting campaign concept. As a DM, my preference before having a player resort to multi-classing is to look at variant spell lists (3e DMG), Cityscape wilderness/urban skill swaps, UA style class variants (which expands on concepts in the PHB and DMG), and third party variants along with 3.0 multiclassing at first level. Once the campaign begins, I think picking up a new class should be more difficult.