Straightback Dirk Proc

[Dizzin]

It seems to me that the dirk's proc only gives an extra attack when wielded on the offhand. Whenever I've had it as my primary weapon, I've never gotten an extra attack with it. So I'm curious why not? It's dice and hit/dam bonuses are extremely weak compared to any other high or even midlevel daggers, heck, enchanted dirks from ZK are better in that regard, so I cant see how it would hurt to get an extra attack with it every 5 rounds or so.

Thanks!