In PvE, you shouldn’t get hit in the first place, or you get hit so rarely that you don’t need to worry about dying. You can still get to ~65 % damage reduction with medium armor, and stay unencumbered, or carry more stuff. The biggest drawback of heavy armor is that it’s heavy.
In fact, I used light armor almost exclusively until they released the Skelos Master armor, which has such great stat bonuses that I switched over to medium.
For thralls, the heaviest heavy armor is necessary because the idiots don’t know how to get out of the way when the Red Mama prepared to swing her tail.