this is because with a staff, you're only hitting every 3 seconds. That means if the mob that you're fighting ends up with 3 hp left after your last swing, you now need to wait 3 seconds to hit it again.
this is actually a rather popular fallacy. because there's no delay when you first begin attacking, you're essentially front-loading all the damage of 2-3 faster attacks. because of this, if the mob/player is left with 1hp and you have to wait 3 seconds to finish them off, you're on even ground with a faster weapon. if, however, you kill them with 3 damage to spare, you're actually ahead of that one-hander.
that said, I actually agree with mal's sentiment. there really arent many situations where you'd want a 2her over a 1h/shield. in pvp, if you're in melee range, you're trying like hell to get out while taking as little damage as possible from a rogue/warrior/paladin. that, or you're fighting a caster, in which case it's not melee damage you're after, but spell interruption, in which case a 1her wins again.
the only time I personally use a staff, is in a pve group situation where I'm not tanking at all.. just playing support/healer/dps.