They mostly are, there are a few cases where they are used though:
1. PVP burst combos, 2h and Fire staves in particular can wack out a massive amount of heavy attack damage in a single hit with the right build. People will often do this and for example bash cancel into a finisher ability.
2. Tanking, if you can get away with not blocking for some time its free resource. Ice staves will also taunt for example.
3. DPS PVE will sometimes have rotations that say, have 1 HA every second cycle on a rotation, when a build has barely enough regen
So it has uses, its just not designed to be the main way to play the game. Combat would be pretty bad if it was all heavy attacking all the time.
Don't forget that there are also proper heavy attack builds, which build around heavily buffing the heavy attack damage.
Xynode's "easy sorc" build is one fairly popular example.