I believe developers underestimate how transaction replacement feature is important, e.g. utilizing this proposal a business dealing with customers bitcoins wouldn't be required to hold private keys with a substantial balance and the Bitcoinica hacks (I stumbled on typing the word "hack" in plural) wouldn't have such devastating results.
Relevant recent IRC discussion: http://bitcoinstats.com/irc/bitcoin-dev/logs/2012/07/11/6#l3981048Bottom line: I think transaction replacement is important, but if we just enable it as-is then I think we open up the entire network to an easy DoS attack.
And I think the incentives for miners aren't right. I think the rule for what version of a transaction will be included in blocks has to be something like "the one that pays miners most OR the one that pays miners first (if there are several with the same fee)."
So I think a scheme where transaction fees are increased, or lock times are decreased, with every transaction replacement is the right way to go.