Why?
Because they are a game breaker, they are the one thing that guarantees you lose the game.
They are only "useful" (should be compulsory, not really useful) once you are winning decisively and you want to get giants. If you buy them to help you push (which is what they should be for) then you give your opponent so much more exp and gold per wave it is insane.
5 Mino's - 165 XP, 20 GP
2 Archers - 66 XP, 16 GP
3 Priests - 150 XP, 99 GP
2 Angels - 100 XP, 132 GP
3 Cats - 90 XP, 198 GP
2 Giants - 100 XP, 200 GP
So as soon as you buy priests, you are increasing each wave by over 40% in XP and almost tripling the gold worth. This is way out of line with any other increase, simply too much.
Pretty much every Cit upgrade is beneficial, this is the only upgrade that is detrimental. It creates a harsh and unfair game rule, where you have to save from buying a useful upgrade, because it will lose you the game.
The XP should be lowered to say 33 XP per priest and then gold lowered to 15. This way the advantage of having healing and extra wave power, is not made completely redundant, because the XP and GP gain to the other team is fair (possibly needs to be slightly lower).
It is like buying an item, that everytime you hit someone, gives them 10Xp and 20 GP, but you have to buy it just to get Ashkandor later on. You won't buy it until you need to.
There is no strategy in using this upgrade, there is only a strategy in not using it, which defeats the purpose of it being an "upgrade".