I like having different friends to play as. Story wise, it gives the world more depth and is good for plots (when there is plots). Gameplay wise, I like having lots of different styles to choose from. Adventure is my favourite game because of this, you expected something different from each character.
I feel the games could benefit from something like this again (not suggesting a sonic adventure 3 because theres enough of those threads) but rather, knowing that no matter who you're going to choose to play as, it isn't going to be the same as another character in the game. And there's a large amount of them to choose from.
Making the characters all play the same and interchangeable with each other (looking at Heroes here) is not good for the game. It just isn't as fun in my opinion and doesn't do the characters justice.