Let us start by looking at what you need for each type of group in the game. After all this is an MMO, it should all be about groups and playing together. If you are only worried about cheap gates of andaron gold by yourself, then you should notreally be worried if your class is left out or not. These numbers are based on experience in various situations and are not hard coded; they change in many ways from situation to situation.
Now that we know what type of characters are in demand, how about the classes themselves. Some can fulfill multiple roles, some have group buffs, some do more or less damage, etc.