Grasp is underated, and it's not a defensive option, it's an offensive option for a tank/bruiser. Like if you take Grasp you should be doing two things to get value: building HP items and trading constantly.
Think of Gnar or Urgot or Darius. Every single time they come up to the wave you should unload at least one auto. And no 2% is not a big gain, but 4% of your HP as bonus damage is decent if you are building HP. And while fleet footwork maybe seems like it's healing is amazing, remember that Grasp is a TANK keystone. So %hp gain on him is going to be more in actual numbers of HP because you are building HP items and may have it in runes, plus you are likely going to get resistance at some point, so the HP you get back will stick more.
This keystone is not designed to allow you to be an amazing team fighter, what it's meant to do is give you a small advantage in repeated trades over and over. I really only ever take it on lane bullies because that's what it does, literally heals you and provides bonus damage on a trade. You wear your laner down over time by taking small, but favorable trade advantages reliably. THIS IS A LANING KEYSTONE. I mean I think Grasp is probably ok on like Mundo too, but that's just because he has no hard cc and just the healing works well with what he already has.
Again, the state of the game prioritizes 100-0 kill pressure and short lane phases. If laning phase was a touch longer, and bullies could pressure their lane without feeling like they are going to get insta-gibbed by a jungle/mid roam, this keystone would be much higher priority.