I can't tank them with Lee like a tank can.
I can tank them well enough to not die, usually, if I hit all my buttons at the right time and with all my Silver Champ/Purple gear. And I may even have to pop an Eruption, Supernatural, or Nimbus or all 3 and toss in an Amazing Grace and a Major Heal bot if things are just being stupid.
It took me 40 levels, gear tweaking, Spec tweaking, and device gathering, and a lot of play time to be able to do what I do with my Unleashed. Maybe I'm just slow but there it is.
As a Tank...
I already know from experience that I can tank Alerts starting at --- Level 11. With Any Trash Gear. And it was easier than tanking with my Unleashed is right now. I never felt like I was going to lose a star, put it that way. Get agro, Regen, get agro, block, get agro, heal, get agro, dps, get agro, take bite from sandwich, get agro. Optional: listen to healer pout with nothing to do. Why can they never be happy...
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Something else, Tanks can steal Lee's agro. Even AT ones. Just ask Todogut.
There is a clear line between melee and tanks already is the point I'm trying to make I guess. As-is, Lee tanks just well enough to make me want to make a tank. And due to he usually steals agro, I'm glad he can live through it most times now!
Don't even get me started on Gravitar.