Cleon
Legend
Yeah, I was just thinking benefits. I suppose we could rewrite it to be a racial bonus and penalty, but I'm not sure it is worth it, as it overcomplicates the creature without really making it a more interesting adversary, IMHO.
Thoughts?
The main problem is that if imorphism only provides bonuses the creature would never be able to completely imorph into a creature with an emulation-worthy stats lower than its own, at least as the power is currently written.
An imorph has a fairly good Will Save of +4, so it's quite likely to be fighting melee-types with a lower Will save who it should be able to emulate. We'd either need to allow it to imorph a penalty or add a line that if one of the imorph's base stats is already higher than its opponent it doesn't need to change it to complete imorphism.
Anyhows, I'm thinking the imorph only uses its shapeshifting powers in self defence, when it expects an attacker to be tougher than itself. It would not use imorphism when facing creatures it knows are really weak - presumably when hunting dire rats it wouldn't bother trying to pretend to be one.
As for it not making it a more interesting adversary, it would encourage the obvious tactic of sending in a squishy NPC to provoke the imorph into taking a vulnerable form ("Halfling torch bearer - attack!"), then the rest of the party pile on to it. I can imagine an encounter when the party may want to use such a tactic, e.g. a bunch of 1st level characters need to deal with a 10-HD imorph.
Evolutionarily speaking, it makes more sense only to have a bonus.![]()
Who said anything about the imorph being evolutionary well adapted. Why do you think they're extraordinarily rare?

The serious answer to your question is that the evolutionary cost of the adaptation may outweigh its advantages. Being able to imorph into a weaker form could help the creature produce more successful offspring, which would make evolutionary sense despite rendering the adults individually more vulnerable to predation.