Grass-types have access to HP Rock/Fire/Ice along with a few other interesting moves (Earthquake, Focus Blast, Sludge Bomb), allowing them to counter many threats that they face. Tangrowth and Mega Sceptile are great examples of this. See where I'm going?
Anyway, what is the general public's opinion on Manaphy? I like its design quite a bit and Tail Glow is awesome. :)
Another thing to consider is that Water-types tend to have bigger movepools than Grass-types, and the latter tends to have quite limited options unless if it's something with a dual-type. Water-types have fairly useful moves and some do learn some interesting stuff here and there.
Anyways, I have never used a Manaphy myself but I know times I used one on Showdown. It's one of only two Pokemon to learn Tail Glow naturally, and once it gets a boost off it can be quite threatening if you don't take it out fast enough. It also learns a lot of fairly good special moves which takes a huge advantage of it.