All content would have to be reconfigured for this.

This would make healing and tanking more difficult while making dpsing easier.

This would make entry to healing and tanking even more difficult for new players and could possibly increase average queue times.

Also balancing, readjusting the balance for this would be a developer's nightmare.

Not saying its a bad idea, but it would take a considerable amount of dev work and the biggest problem is would the community accept this or not.