Well obviously it's true that you CAN tank or heal a scenario. You could just stand around twiddling your thumbs in a scenario too. Most people play a support role because they want to be useful, not because they want to be carried or useless.
Tanking and healing is either required or it's useless. That's the game design. A tank or a healer in a scenario is just going to slow things down. It does not feel fun to me to "heal" a scenario.
This increased importance on areas of the game where healers and tanks are "optional" worries me. Especially because damage dealers get tons of tools (cc/self-heals/shields) so they don't need tanks/healers but healers and tanks don't get any significant increase in damage on their own.
They should start putting a warning label on the specialization screen. Damage dealers get to excel at every area of the game they take part in. Tanks and Healers-- not so much.