It's easy to confuse spec for playstyle.
-If you have a dps spec (aka carnage/gen): DW 99% of the time. The 1% is if you want to go 2H to perform an IFA
-If you have a tank spec (aka brute/gen): the most effective thing to do is 2H 80% of the time. The other 20% are for BB (every 10 seconds) and breach (situational).
HOWEVER you can run a tank spec and never use DW combos. Not as efficient as the above but it works. You can also run a tank spec and use DW combos 90% of the time....but guess what? It doesn't work so well ^^