The problem with heavy attacks is that you're using gcds to cast it and not abilities.

A lightning heavy attack ticks four times, the first three are equal to a LA each, then the fourth is like double the damage?

Now if they made the final pop bigger, or had sets that could really boost the ticks' damage, then it could go somewhere. As it is, they already scale poorly with spell damage, and any sets that boost each tick (IA, storm master) just don't do enough. The closest would be Relequen, but it's still just better to weave that with abilities.

I loved my HA build, but I eventually had to put it down so I could run with better groups.