I use slower daggers when I PvP and I can't see any other reason not to. If your target is living long enough for a faster dagger to mean anything DPS-wise you're probably doing something wrong (or probably already dead). Since your goal is to kill your opponent before they get a chance to react I would say two slow strong daggers with high damage are much better than faster ones with comparably lower min/max damage. People used to roll with faster daggers for spell interrupts against casters, but since Blizz changed how interrupts work (now only the first two blows have any kind of effect on the spell) I don't see the point any longer. Even then the best way to interrupt someone's spell was to kill them and two slower daggers should do a much better job at that since Muti is all about burst damage.
That's just my
![Smiley: twocents](//zam.zamimg.com/i/smilies/twocents.gif)
though.