Specifically, I'm thinking of it as a substitute to Leech Seed. In one way, it's definitely less effective, due to the fact that you can heal opponents, and depending on what you're leeching, you'd probably heal less from Grassy Terrain, but the fact that it would make Giga Drain 50% could definitely make up for that.
Either way it takes a turn to set up, and at least with Grassy Terrain it doesn't disappear if they decide to switch out. The main benefit here would be the Grass-type move boost. The fact that Venusaur has ****-all for coverage with its offensive moves means that it's probably not a bad idea to boost one of the 5 types it actually has access to. --- 3DS Friend Code: 0361-6447-6950