Actually, I think Superforms should be in COOP, but NOT Single Player.
How many times do you see players who ALL the time played as Knuckles, Tails, or any custom character switch to Sonic ONLY when all the emeralds are there? It's extremely annoying to see everyone switch character to Sonic just because he can now go super. Face it, super isn't anything special. SEGA made it that way, netgames made it that way, the amount of time most of us have been playing this and many other Sonic games made it that way.
In Single Player, you're forced to be Sonic if ya choose his skin, forced to take his challenges. If Tails can go Super THERE, there's no reason to ever play as Sonic and collect the emeralds if you can do it the easy way. In netgames, one way or another, there'll be Tails and Knuckles player switches, setrings, whatever can be done to EASILY get the emeralds, since you don't unlock anything anyways. THEN they'll switch to Sonic since he's the only allowed to go Super.
In Single Player, people should have a motivation to be a harder to use character, since you're forced to be that character for the rest of your quest. You CAN't switch to Tails anytime of the day for 1 second to get past a hard obstacle. Super Sonic IS the motivation for players to be Sonic in the first place.
In netgames, people will be lazy anyhow with the best character. And once Super Sonic is unlocked through easy Tails or Knuckles emerald collecting, they'll simply switch to Sonic for the mere purpose of Super Sonic, not because they're a Sonic player.
I say coop gets a superform for all, and single player maybe a devmode cheat or Pandora's box thingy or something to play around with. Super Sonic isn't that special anymore. SEGA made it that way, the amount of supers in ANY Sonic game made it that way, the amount of time we've seen and been Super by now, and putting Super in match and CTF mode helped to it. It's not making Super Sonic look special by making Super for Sonic-only, it's just making the game feel limited.
In 1P, keep it as reward for playing as a harder character. In anything else, it makes Super Sonic seem LESS special if he's the only one. Imagine 6 Super Sonics in 1 netgame, after all. All thanks to Tails and Knux not having a superform. Real special and rewarding, considering 5 of those players were Tails and Knux ones before.