The harp is a lie.

[Scardon]

But what's to stop us from playing a class in a group? Why play a RNG or BRD in a group if I can instead be an ARC with MRD and GLA and LNC and CNJ and THM skills?

Because apparently there design goal is to change things in such a way where jobs will be infinitely more useful in a group than any combo of class skills. Even if someone does find a useful combo of class stuff they can just gimp the hell out of it until its worse than all the jobs. At least this is the intent I gather from all these dev posts.