The points made - no saved stats, no reason to encourage knifing - are all good reasons why dogtagging should not be included.

Personally, though, I think that meleeing a foe should have some sort of alteration versus normal kill, depending on what's possible. Having the target not make a sound when dying would be great - knifing would be akin to 'black ops' assassinations, where stabbing someone would simulate a silent kill. Ideally, they wouldn't provide a global death message, although stopping that probably borders on the 'impossible' aspect. Right now, grabbing your Vibroknife is the opposite of stealthy - the other guy is almost certain to hear it, and they have a chance to blast you.

At a bare minimum, either silence the melee weapons, or else add in the option to have a silent knife. Use the Customize pane to select which weapon you want - the strong, noisy one, or the silent one with a stab instead of a slash.