Letting ppl select to use addons is like asking for someone to alter the direction of your playing experience.
If they start using them, you will have to do so in order to not fall behind.
Using them, makes the game easier, not using them = more time spent on content (taken that the content is challenging at all from the start).
Let the hardcore part kill **** when its a challenge, without using meters, then nerf the encounter if you are to listen to the whine from a few areas in the world. Then non needs meters, both the "good" and the "not so good".
Having addons or not, makes a huge impact on the game.