Why are replacement transactions currently disabled?

[adhoc]

This question is mainly directed at the bitcoin developers I guess, but why are replacement transactions turned off at the moment? They seem like a great feature that could open bitcoin up to various forms of trade which could only help the BTC economy.

Thanks

[adhoc]

also, if the people who might know the answer don't read the newbie thread, would someone please upgrade this thread? I don't want to be pushy but I feel like replacement transactions are a really good mechanism and don't see why they are disabled.