the higher the cd spec the higher the chance to hit with the off hand.
haste effect applies every time you hit with the off hand
when only hitting with the main hand there is no haste effect
since the cd/dw formula got buffed here the off hand chance to hit is alot higher than on live and since there are no cele procs you always want a slow main hand and a very fast off hand
there is just one exception: when you are a ranger with nearly no cd skill but high pf skill, dmg is higher when you have a medium speed MH and a slow OH but i can not recommend this tbh