It reduces the functionality of zero confirmation transactions.
No it doesn't.
If I am a merchant should I reject RBF?
You should wait for confirmation if your risk model says you should. This is already the case, and many properties of a transaction (like low fee or spending unconfirmed inputs) will make most zero conf accepters wait (and still they frequently get ripped off; because the Bitcoin system itself does not provide security for unconfirmed transactions). Fortunately, Opt-In RBF at least means that there is no risk that the confirmation needs to take an excruciatingly long time.
Even Gavin Andresen is against RBF and last time I checked he was still a Core developer, so now you do not even have your all important "developer consensus".
If he is, he failed to speak up. Of the 19 people that commented on the PR in the nearly two months that it was open, none said anything negative.