haha yeah I know it's rediculous!
Personally that excuse is a load of crap to some extent, because they could make a fantastic game by designed an awesome level, and then having different routes through it for Sonic, Tails and Knuckles.
Sonic runs and grinds, Tails has flight paths and Knuckles climbs, glides, digs and punches his way through a level.
They could change the gameplay by having Sonics level as they are now. Tails' could be a race [like they are in Sonic Adventure] where you have to hit all the shortcuts to win. And Knuckles could have to deviate off the paths slightly to hunt out say 3 emeralds in each level.