Well, Boeing nor AW is shipping directly to Afghanistan now, are they? If you need to keep your birds up there, and it takes let's say a week for the parts to come from the UK or NL, provided you put them on the next flight, it's better to take the part from another aircraft and ship that, and order a new part for the cannabalized airframe in the mean time (it could be going maintenance anyway). You're fighting a war several thousands of miles away. It's a good short-term measure, but the number of instances certainly show that the logistics chain isn't running as smoothly as it should and probably that the wear-and-tear is tremendous.
You have bought more than you need for a reason.
We don't know how long it takes for the part to be replaced on the victim. We don't know the condition of the victim. We don't know which parts are concerned, long-lead items?
It has nothing to do with the price, actually, it's the other way around. More expensive, the more parts, or higher quality, or more complex parts, you are dealing with bigger logistics chain and longer leads. You just need more spare parts, if you run out, it's time to use your spare aircraft or use their parts.
UK: 67 ordered, 8 delivered, 59 built, I think I've read about one incident which resulted in a write-off? Don't have more info. The British machines have RR engines, which seems perform better in hot-and-high conditions than the GE ones, Longbow is carried (Americans uninstalled it for Afghanistan). Field mod: Instead of full capacity of rounds, they've used some space for additional fuel.
NL: 30 ordered, (12 interim AH-64As lend), 30 delivered, 1 written-off in Afghanistan crash, leaving 29. Of which 8 are stationed in the US for training, five deployed to Afghanistan. That leaves 16, now 1 was damaged in the powerline incident, 4 needed repairs after Italian fueling mishap, that leaves 11... well let's say 8 are in operation in the NL at any given time at the most. Note AH-64D, without Longbow, the add-on was never bought. Turned out to be quite convenient as it's not really needed in Afghanistan and saves weight. They do have received a countermeasures upgrade though, basically bolt onto the wingtips, so Stinger carriage on the tips is no longer possible. Then again, those A-A Stingers weren't bought either I think, we do have the MANPAD Stingers in operation. Not sure how it works, compatibility wise.