I asked the same question.
The answer lies in the fact that after the windfury effect goes off, there is a hidden 3 second cooldown on the ability. So even though it says you have a 20% chance to have a windfury attack, if you swing during the cooldown you can't "Proc".
Also, since the extra hits are free of a time component, you want a weapon that does a TON of base damage. Thus a weapon that hits really fast for little is inferior to a weapon that hits slowly for a LOT.
Hope that helps