If you kept the limited cross class slots (5 max), remove the restrictions and redesign the abilities that get cross classed, using traits to buff specialists so they don't get overshadowed, this would work fantastic and avoid most of the doomsaying in this thread.

But we need to REALLY pick and choose what skills should be available to cross-class. It should bring a little of that class' ability without overshadowing what they do. PLD's can be Flash, since it still works as a threat generator, but it's got to be that PLD's is much better and they have better mitigation for the job. Cure or Cure II can work great, as long as WHM still gets all the better heals exclusively. I'd say each job should bring to the table up to three core cross class skills and two core traits. Tops. Your 5 slots must decide what skills or traits to use.

Yes, there will be FOTM builds. But you know what? Other games deal. Devs change things. Eventually a good enough balance will arise.