Each legendary in the old ones had a story behind it.
Except they didn't. The birds, literally, were just sitting there in Generation I. Any semblance of a backstory they might have had was from the anime, which counts toward very little.
The rest of the trios were there just cos I guess.
The Regis, the pixies, the kamis and the musketeers are all given stories within their respective games.
but lots of the other just don't.
Likewise for Heatran, Cresselia, Darkrai and Shaymin. All of them given more story or explanation in game than the birds.
They don't need to keep adding 2 or 3 to each generation.
Still we use "need" in the criticisms, despite there being no way to objectively define "need".
And this is why I don't play B/W.
But you're perfectly content to complain about them.
My best answer probably stems from his faux-alien (round head, full colored eyes) appearance, and the cannon on his back having no obvious physical purpose but to simply outfit him with a weapon, while we already have Blastoise playing off the massive space inside of his shell and the water pokemon's need to be continually hydrated to use his as example of making the silly plausible. Genesect was simply built for battle, and it shows. While Team Plasma being responsible for the cannon (Checking Serebii, there seems to be a touch of inconsistency between the White and BW2 Pokedex entries on whether they added it or changed it) would justify the sloppy addition, because than it really would be, until I suspect Genesect was designed explicitly with that backstory in mind it doesn't clear it.
Well. Thank you for explaining all that, at least. The BW2 entries say the cannon was added - we should probably go with that, given that a prehistoric insect Pokémon with a cannon on its back would just be
too weird for anything.
What I would ask, though, is that if Genesect being designed with its backstory in mind would be that big a deal? Assuming you are in fact making a point of that - I'm not quite sure from your last sentence. Can't be likely conclude that Mewtwo was created with its backstory, being a monstrous clone of Mew, in mind as well?