Ok, so my Stalker attacks some group of +4s, full of bosses. My damage is pretty good, but I can't 1-2 shot +4 bosses. I have 50 Def, and pretty good resists, so the whole alpha comes in and my green bar doesn't move. Then that Sidekicked Blaster behind me throws an explosive blast. All those mobs I just aggro'd ignore me and attack the blaster, since I don't have Taunt. Or any Taunt attached to my attacks. Or Gauntlet. Or a Taunt Aura. Or even a level 4 threat level.
So I didn't really "Tank" anything there. I just took the Alpha.
Which on a lot of teams is all you need. Maybe our team had a FF Defender or whatever and the Blaster is fine. But a Stalker can't really "Tank". That is all I meant.