Don't hammernators usually mix some lighting claw terminators to help slice trough infantry?

Well against a squad of just hammernators pariahs could be quite good yes, but then it depends heavily on your metagame. Against horde armies or even stuff like blood angel's assault squad spam, I don't really seem them doing much.

Heavy destroyers on the other hand are an excellent offensive choice, and the necron's only reliable way of blowing up vehicles at range.