Its average in the sense that it take turns to hit.
so if you have 1000 dps MH and a 100 dps OH it will do really bad damage 50% of the time.
Imo shield with a top notch 1hand is the way to go.
But it's not the actual average in the sense that they don't actually take turns to hit in practice. The stat calculation is misleading because it is calculated off the fact that while autoattacking, you alternate between main and off hand, meaning that 50% of the time you WILL be dealing !@#$ty damage.
In reality though, all our generators go MH, OH, MH, which means that you'll only be doing %^-*ty damage 33% of the time, so while the stat sheet may not reflect it properly, you don't actually lose that much dps when off handing a crappy weapon. Still true that you lose dps though.
So anyways, my point of this was that sometimes it can actually be pretty hard to determine without a pen and paper what the net dps change swapping from a suboptimal OH weapon to a decent shield will be. it'll look good on the paper doll, won't necessarily be the case in game.
Not to mention there are other factors to take into account as well, like the IAS affecting stuff like proc rate for crits, LoH, etc etc
Don't get me wrong, I have nothing against wearing a shield, I slap it on too when the situation calls for it, just pointing out some things :P
EDIT: turns out i have to check to see which tabs i opened two days ago and haven't read before replying to it, sorry for the necro...