Hey, Simisage isn't that bad. The only bad period it has is the one between Vine Whip and Seed Bomb (13-22?) - I agree that at that point, training Pansage is an absolute pain. But once it learns Seed Bomb, it only gets easier from there - you get Dig, then Acrobatics, then Rock Slide... and many more. I evolved my Pansage right after it got Acrobatics, and even though it had a horrible nature (Bold) it kept being a major powerhouse right until the very end of the game. (final set Seed Bomb / Acrobatics / Rock Slide / Shadow Claw)
Now, if we're talking about bad Grass types in BW(2), look no further than Cottonee and Snivy, aka "I can't damage things to save my life".