The more I think about it the more issues I find with single spec classes and I just don't really see what they fix. Imagine you have always wanted to be a bard and your best friend has dreamed of being a chronomancer, so you both main them. You level up together, slowly, since you're both healers and cannot switch to anything else. Then you decide you want to do a dungeon. Sorry, you cannot do that, because neither of you can swap to a dps or tank spec. You can never play together in any content that doesn't allow for two healers. And what of tanks who like to play with their friends but HATE tanking random dungeons for strangers because of the shitty behaviour in such groups? Sometimes you just want to switch to dps and not be responsible for the group (same for healing, sometimes you just want to do some content and not be responsible for the group).
I just don't agree that single spec microclasses solves anything. The more I think about it the more problems I see it causing, such as with gobbling up more art resources than a single 2 or 3 spec class would, and social and gameplay issues caused by never being able to be anything other than just a healer or just a tank.