If the characters are going to be unique individuals, then it's inevitable that some characters will be better than others, thus tiers are formed. Giving all the characters different moves and different priority in there moves will make the characters unique, which is very important. Without these principles, a fighting game becomes boring and stale. It's impossible to have diversity in the roster if every character is going to be equally balanced, and that's a major reason why tiers are needed. For example, some characters will be faster than other, but another character might have more stamina. Or, some characters will have alot of range, whereas other characters will be more melee based. It's important for a fighting game to have a diverse cast. If not, the game will not as fun. Balancing all the characters out is not going to work.
That is all very true and correct. However, the developer should not set out to create the tiers before the game is released. They should be trying to make all the characters equally useful, and different. Inevitably some will end up better than others, but that should be entirely unintentional save for instances of joke characters (I am looking at you, Dan). So to say Sonic should be a high-tier character is bad. The developers should not create a tier system because such a system should not exist. Whoever ends up being best should be accident, not design.