They cost a fuckton, though. It costs you Spell Level x Caster Level x 1800 gp. So to be useful at all, you'd sink all your resources into it (but if you got, say, shapechange or gate, you'd be pretty win forever and ever). It's really a binary I win/I don't buy it situation. It's either gamebreaking or worthless because it costs too much.
Sometimes, I think it'd be easier if the DM just priced it however they wanted. Then I remember that that sucks too. So it's all pretty weak.