Quote from Viisha
Since most damage in this spec comes from the tornadoes (80%+) the offhand damage can be neglected and is used primarly as a stat stick.
Hope this clears it up for you.
I haven't tried it much myself nor have I made the calculations but I would never have thought those tornadoes could represent 80%+ of the total damage output (is that what you meant?). It sounds pretty crazy to me. At first, I thought it was used more as a way to proc more crits, hence generate more rage, get a longer berserk, etc. then get some more damage as a side benefit.
That still sounds curious to me, but if it really is 80%+, then, of course, it makes a lot of sense.