They got disabled quite early and enabling them one by one is a soft-fork in many cases. Also to trade one needs a order book - are you really suggesting that the whole order book for several decentralized exchanges should be stored in the memorypool of Bitcoin clients?

I am aware that Bitcoin CAN work with scripts (and in some cases it might make sense to represent IOUs with Bitcoins). The trading + order books of the represented IOUs however should likely happen outside, either with blinded tokens on dedicated servers like OpenTransactions suggests, on a completely new alt-chain (several ideas flying around, never heard about actual implementations - maybe Namecoin could be viewed as one), on Ripple or a fork of it or on a centralized exchange (GLBSE, btct, bitfunder... you see how they end up!).

If you use BTC to issue IOUs (no matter if Mastercoin or colored coins), you anyways have the "trust issue" that a lot of people around here don't seem to like about Ripple. The only thing that's left aside from requiring trust (which any of these solutions will need) is that it is not sure if Ripple consensus really is as safe as advertised. Contrast that with the known problems/limitations of MasterCoin and colored coins and you might get why I suggested it in the first place.

Also it works _right now_ with a working open source client and server, order books, trades etc. as opposed to a lot of other technologies presented here which are in the best case just starting up with some funding and in the worst case some forum ramblings of people who think "a p2p exchange would be a nice idea, someone just has to do it!" (Edit: like the OP who re-invented colored coins).