I'd wager not many want to see a white mage tank or a warrior heal. You're basically asking for roles to be eliminated.

You know what you signed up for, or at least I hope you did. This game is about choosing the role you want to play first, and your class second. And you choose the class that you feel will provide the most enjoyment in that role. What you suggest will recreate the subjob nightmare that was FFXI, and jobs not being played as intended. The small number of jobs for roles such as healer and tank is not the issue, and the more jobs capable of filling those roles the harder it becomes to balance them.