Realized I had more to say than just responding to people wondering if it would work...

If they can pull this system off properly (it is a lot of work to balance) it could be a big step forward in balancing in the MMO world. Typically games have a difficult time of maintaining balance across all their content, because of the inherent strengths of different types of classes. For instance, a specialist will have an easy time doing large-scale PvE content because that's where they shine, but if they are too specialized soloing may be utterly impossible and they'll end up needing help to do everything. On the other side you have a jack-of-all-trades who can solo even some things intended for groups and has tricks for any opponent in PvP, but no one will invite them to any parties because they can't fill a specific role.

You can see this in FFXI (or could at least, I haven't played since the add-on BS) almost to an extreme. Jobs like bard were massively beneficial to parties and thus got invited to everything, but they needed help with everything that could "normally" be done solo, while jobs like beastmaster were amazing soloists but never got invited to anything. This is also the reason WoW introduced "dualspec," allowing people to choose a more well-rounded build for PvP and a more specialized one for PvE (generally).

If classes are still good generalists and jobs are true masters of one thing, and they level up together, you'll end up with an automatic situation where people have one solution for solo and another for big groups. In this way content can be balanced separately, and you don't have any of one bleeding into the other. Characters will neither be so similar there's no point in picking, nor will they be so different they're helpless without a group, because the player will always have both options.