Hello.
http://wowwiki.wikia.com/PoisonsWeapon speed Edit
Faster weapons do not always increase the rate at which you apply a poison: unlike the other poisons (which have a fixed chance per hit), Instant and Wound Poison use PPM (proc-per-minute) mechanics. This means that even on a very slow weapon your Instant or Wound Poisons will add the same amount of DPS (damage per second) as on a fast weapon. Note that without Improved Poisons, Wound Poison's DPS is higher than Instant Poison's due to its higher PPM.
The argument for using slower weapons is this: based on your weapon speed and your poison's PPM, an average proc chance is calculated that will be used every melee swing. However, your special attacks (Sinister Strikes, Mutilates, etc.) can also deliver your poisons, and they use this very same calculated proc chance. So equipping a slow weapon results in an increased chance of applying poison with your special attacks, which, of course, are not restricted by your weapon speed. The difference in damage is rather minimal though, so people often just follow the "slow MH, fast OH" because they could always put Crippling/Deadly/Mind-Numbing on their OH without much reduced effect.