As an exemple without spoiling anything when I had my melee tank with my Agent (Sniper), I had more damage mitigation from armor than him ... without being a tank myself, I'm a better tank than my tank companion, no point using him.
While minimally spoiling, the Agent's melee tank is not male. The male melee character is DPS and is not intended to have high armor rating.
I have troubles with most tank companions. The survivability buff does not outweigh the damage output advantage of DPS companions, at least not for my preferred combat pace. Lord Scourge I love just for the purple AoE life-sucking. In fact, he and Khem Val are my favorite tanks just for that, because it looks awesome and scary. I gear them and take them everywhere I can. That said, when the going gets tough the tough takes Doc out instead.