I've thought about this, since it's an obvious concern with people playing ranged more due to their ease and lack of positionals (in most cases ie. MCH, SMN, RDM).

However, here's why it wouldn't affect anything:
  • Duty Roulettes automatically match 1 melee and 1 ranged in a light party (unless someone queued as a pre-made, that's the only reason you'd have ie. 2 DRG in a dungeon).
  • High-End duties have mechanics clearly designed for 2 melees and 2 ranged, so many competent PF leaders set the slots to 2 melee and 2 ranged.
  • The 5 percent makes you look for at least 1 melee, 1 magic ranged and 1 physical ranged. And whatever effect they have on LB generation now.
  • Of course, you will also want a melee LB3 if it's not needed for a different purpose.
So for all these reasons, I don't believe people would stop inviting melees to their raid parties.

And where was similar concern over people taking only 1 tank, or sometimes 1 healer as well, to certain Extreme trials such as EW EX2? There are even people doing that with Unreal. That seems a bigger likelihood than for DPS jobs.

All they really have to do to solve a problem like this is make an 8% instead and you lose a % if one of them is not of the right role (ie. not taking 2 melee means you lose 2%). Or add key utility to each role that is unique and needed.