You give up corporeal hardening (25%), calcification (25%), and possibly petrify flesh (25%?) in exchange for special dodge/evade.

Now just having corp hardening and calcify up is a flat 43.75% dam redux (at the cost of flight).

So in order for malliability to be equally usefull it would need to fire around 1/3rd of the time on melee hits or at least just on skills (which even with arial or drow I don't think comes close to that high).

I don't think it is coincidence you really only see scions using this spell heavily for skill avoidance over general survivability. Without despoil you just don't have the HP base for the gamble to pay out often enough.