Ya, lots of mobs with 125-150K health and you don't get group XP bonus to boot. (or is XP cut by 1/2 also ? I don't remember.) I can guarantee that the mobs / bosses don't give 25X-50X as much XP which is how much they'd need to be in order to even come close to what you can get in a normal instance.
It's a neat idea but totally unworkable, feel free to try it and tell me if I'm wrong but I am 99% sure I am not.