I think the consolidated one button might affect people like myself who tend to mash button to queue skills to minimize latency effect.
So for example I'm in the middle of DRG rotation ....-FAC-SSD-TT-J-VT-...., usually I will mash the TT button after weaving SSD to queue it.
But if TT and VT are one button, then if I accidentally mash that button then the VT will get queued, and it'll become TT-VT-J instead of TT-J-VT and mess up the rotation.

For people with high ping like myself, pressing a button once and waiting for the GCD to pass before pressing another button instead of mashing them will significantly affect my DPS. Which is why I don't raid as NIN because I can't mash buttons the mudras especially during TCJ and have to press 6 times one by one slowly to avoid the bunny >.<

I don't really play PVP, so I'm not sure how it works there for melees.