Class systems by nature are arbitrary in their design. Most class definitions come from tabletop systems (D & D, T & T etc) . A person set down and decided how to differentiate the classes. It was based on what the person or group of people thought along with some reference to history. Other people continue to use that system in other rpgs and crpgs.
It was simply an easy way for people to get into roleplaying. A person can say I want to play a warrior. The person could look up warrior in the guide and see what that person or group of people thought a warrior was like. The gamer has a template on what is a warrior, mage or rogue.
A classless system requires more work and explanation to be better at designing the character or party. Even in a classless system gamers still tend to create the same basic classes. The classless system allows more flexibility in the creation..
Check out the thread I made here: http://forum.bioware...to-all-classes/
The thread discusses the pros and cons.
Bioware could implement a classless system with basic templates for gamers who want the basic classes while allowing for open skills and abilities.