Hmm... well that kind of renders Crushing Surge to be nigh useless if it doesn't stack, because it doesn't give you enough temp HPs to be meainingful.
I don't see that at all. At will powers aren't supposed to be amazing. They're supposed to be basically a melee basic attack with an extra little bonus. Preventing CON mod damage is roughly equivalent to doing an extra STR mod damage (Cleave or Reaping Strike), or pushing/pulling 1. There are arguably better choices, but it's hardly useless.