MerricB said:The things is... these items aren't for just high-level adventuring.
Having to take two move actions to change this over is a big opportunity cost. Instead of going "Charge, full attack" you are going "change + move, change + single attack". That's half the combat gone already.
Why are you changing them in the middle of a combat? If you know the dungeon you're going into is mostly undead, you stick on your undead-killer crystal and forget about it. Duh.
Note that part of the reasoning behind the recosting of magic items isn't just about the GP cost of items; it's also the actions needed to activate them and the slots they use. It sounds great having five of these gems available to add to your weapon, but it's effectively the same as having five hats: you can only use one at a time. So, a certain portion (and generally a significant portion) of your money is going to be tied up in these very situational items.
Crystals have the benefit that their cost increases linearly, rather than quadratically. As long as that's true, having a clutch of them is going to be cheaper than trying to get the equivalent enchantments on your weapon. They thus have the advantage even if adding and removing them takes significant time.