*Ponders*... I think "money sink" should always be a secondary objective of any mechanic. Just for what that's worth.
And three things:
1) Durability shouldn't ALWAYS decrease sheerly with weapon/armor use. I think it would tie into the weapons-vs-armor-types system very well. Use your sword against some plate armor... degrade your sword blade. Use a maul against plate armor... probably doesn't degrade your maul. And vice versa. If an enemy with a rapier is attacking your plate-wearing Fighter, then there's almost no way his armor's going to degrade. This actually gives you tactical means of affecting item degradation.
2) There needs to be some actual positive incentive range for durability. Maybe everything above 85% actually provides new bonuses/effects (like bleed damage/chance, cripple/impale chance, etc.)? Maybe weapons you buy in shops start at varying durabilities? But, when the best-case scenario for item repair is "you go to spend money to stop your stuff from sucking," there's not a whole lot the mechanic is providing to the player. Even with HP, having it be high means you can take a greater hit without dying. But there's no significant durability damage variance, so you don't even have that with durability. Add to that the fact that HP doesn't always cost money to regain. Just for comparison.
3) Unless you're just constantly thwacking rock walls with your blades, and diving upon jagged boulders in your plate armor (see point # 1), durability should take a while to significantly degrade. Complete item breakage, if it is included, should be QUITE rare. In fact, instead of any kind of effect simply having a chance to break an item, I'd rather see a very tiny chance for something to deal 10-times normal durability damage (like a durability critical hit). Kind of like what they did with missing. There's still a dynamic as to how well you can hit in combat now, but there's much more nuance. You're rarely going to have a 30% chance to completely miss someone, or score a critical hit. Same idea with durability, only, the numbers would be even MORE different, because the expected, abstracted frequency of you just getting your armor torn in half or your sword blade cracked in two is even lesser than the frequency with which you expect to land a critical or suffer a miss in combat.
And, because it was mentioned some, I got to thinking that it might be interesting for magical items to have some sort of durability. I don't think permanent, passive enchantments should lose anything (unless maybe the item breaks, but that's more because you can't wear the armor/wield the weapon anymore, feasibly, and not because the enchantment just somehow doesn't work anymore). Honestly, I think it'd be AWESOME if your sword blade snapped in half, and you could still fight with the half-a-foot jagged nub of a blade to much less effect (almost like fighting with a broken liquor bottle in a bar brawl or something, heh), and STILL have that nub deal +5 fire damage, but without your swords initial 5-10 damage or whatever (maybe it's 1-2 now?).
But, ACTIVE magical items (wands, staves, things with charges, or with toggleable magic effects, etc.) could suffer their own form of durability. It would be more temporary, time-sensitive durability, though. Sort of... magic focus/channeling fatigue, if you will. That staff casts Chain Lightning? Great! Maybe the first cast fires 30-damage lightning, and jumps to 6 targets. If you cast again right after that, maybe it only fires 27-damage lightning that jumps to 5 targets. And so on. However, if you wait 20 seconds (arbitrary example duration), THEN cast again, it's able to produce maximum-efficiency Chain Lightning again at 30 damage and 6 jumps. It kinda works the same way as some like... I dunno, invisibility ring. Maybe it only makes you invisible for 3 minutes, then it deactivates and goes onto a cooldown. Well, with Chain lightning, it's kind of like you're activating an effect, then immediately deactivating it.