Whether or not the priority to get in beta is to be legacy or someone who hasnt played the game at all, really shouldn't matter. In fact, people who are currently playing the game would prove to be better testers as far as testing for bugs, defects, etc.

People who haven't played FFXIV would be better for testing the game as far as story, graphics, and explaining the new game and spreading the hype, if the game does prove to be exciting enough for a hype.

Current players do have the ability to explain what is new and what is carried over form 1.0, but to people who haven't played FFXIV, it is all new to them and it is better off being explained as a new system.