Oh, the problem is rarely the ship's crew - they know all too well that if something breaks and they haven't got a spare, they're up shit creek.

The problem is rather - I'll just let my prejudices roam freely, my apologies - beancounters who realize that it is a waste to keep X sets of spare parts on X ships, when they are hardly ever used - heck, much more cost-efficient to keep one set of spares in a common location, then dispatch as needed.

Except, of course, that 'dispatch' is non-trivial if you're, say, doing core samples along the mid-Atlantic ridge...