Melmoth has a point: Item durability is more or less a meaningless irritation at this point in MMOs.
Does armor and weapon durability make you play or think differently? Would you change tactics because of it? Strategy? Class makeup for an instance or raid? Would item durability make you go somewhere that you normally wouldn't? Well obviously this last one is true, you'll hit repair vendors more often, in the city that you're sitting in anyway. So did anything change? Nope.
Durability is a gold sink. That's the given justification. Makes sense. Or does it? I don't think so. If the purpose is merely to regulate the gold supply, why not work off the supply end of it? People might complain if quest gold went down, but would they notice or care about a few copper shaved off mob drops and trash vendor prices?
Repairs could also be WoW's version of a death penalty, a way to discourage dying. Except for that they are too low. And redundant. We already don't want to die. Proof? Battlegrounds. People avoid dying even when armor takes no durability loss (beside the combat damage, but no death damage). People still raid and wipe for hours, going through full durability cycles over a night. It's not the repair cost that makes them play carefully and cautiously, it's the time cost.
Repair costs simply are not adding anything worthwhile. The gold sink could instead be less gold added. They don't discourage death any more than already existing mechanics. They don't send us somewhere new in the world. Instead they send us to the same vendor across the street from the auction house to hand over some more gold. Sometimes they waste our time in a heroic when gear breaks, then we yell at the thoughtless idiot who didn't repair before joining. Repairs don't add any meaning.
This doesn't mean that they couldn't. Repairing armor could be a part of the player economy, using player crafters or even player materials. Is a cobalt bar any different from a few gold? Yes. Yes it is. A cobalt bar sends someone out into the world to find a node. A few gold sends them to the same daily quest they did before. Even worse, when we're saturated with gold and gold sinks, the devs think we're becoming poor, so they add more gold, and then we're inflated so they add sinks, in a round and round cycle of running the same daily so we can pay the same armor vendor from whom we never buy anything.
I suppose this is subjective. Maybe you think that repairs add nothing but irritation, but you also think that running to the black anvil added nothing either. And maybe someone else thinks that repairs make everything so much more real. But I suspect that last person isn't very common.