Almost all tanks will be getting almost no sustain from that. If you're at 75% health, you'll have 2.5% lifesteal against minions. At 50%, you'd have 5%. For someone without high autoattack dps, that's almost nothing. It's not enough to keep me in any lane that grasp wouldn't also keep me in, and it will lose me some lanes that grasp could have won. If I don't have grasp, my trading isn't as efficient, so it's easier for my opponent to bully me. Additionally, it becomes completely useless as the game progresses (whereas grasp stays useful). These are not insignificant costs.
I wouldn't buy lifesteal on a tank, and I won't take lifesteal in a mastery on a tank.
Meaning, I don't even have to trade with someone to sustain, with grasp, you do
You're not sustaining enough to keep you in a lane where you would be unable to trade, and if you're not trading at all, you're losing no matter which mastery you have.
Seriously, I can't think of a lane this would help me in. Usually if I'm losing on a tank, I'm not hitting creeps much and I'm farming with abilities or under tower. If I'm doing something else, my enemy is incompetent and I'm not losing to them in the first place. The mastery wouldn't give me anything in those situations, but that's also the only situation it gives me any benefit in because the lifesteal isn't present when I'm healthy and I don't do enough autoattack dps to sustain myself in actual trades. I see no benefit, except in situations that don't happen.
I'm sticking to my guns. Virtually all tanks will not use this mastery. I do not have a single doubt about that statement.