I think it would be nice to have the option to turn off XP gain for those who don't want it. If you like getting bonus XP, (I'd fall into this camp btw) you won't be affected at all.
With the stat squish coming, levelling is going to get even faster in the near future and if being able to slow things down is going to please some players and make them more likely to stay subbed, then I can't see an issue.