Ummm I'm pretty sure you answered the second part of your post with the first part. Shadowbolt is so weak, BECAUSE there are so many other sources of damage; add in to that the effect of Metamorphosis, and it's pretty easy to see why Shadowbolt has to be balanced to be such a weak nuke. Does it suck for PvP? Yes. Is it fun to have such a weak nuke? No. Is it necessary for number-balancing purposes? Yes.