Doesn't Grassy Terrain also heal the enemy team? Lol
Do walls really care a whole lot? They just want to stick around a while. Though, I imagine there are a few walls that may care. Also, it does not heal anything with levitate or is flying type. I dunno, but it's an option and I'm not too concerned about it healing the opponent so long as I'm healed and am able to stick around.
A lot of sweepers are too frail to benefit too much from it, though there are some bulky ones I suppose. The buff to Venusaur's Grass moves kind of mitigates the healing. Again, it's an option if you're dead set on giving Venusaur some passive healing.