I can understand their concern about class stacking. It's to prevent having 2 Paladins, 2 Warriors, 2 Bards, 2 BLMs, etc.

For healers, it's not a big deal. In any content that matters you generally see groups running with WHM/SCH.

Tanks is another story. You often only see double paladin as of 2.0. With the 2.1 changes, Warrior got a huge boost not only in tanking capability, but they got BRD's RoD debuff AND can increase slashing damage. You want to take a Warrior for variety.

DPS is hit or miss. It shouldn't be hard to find varied DPS in the game for coil groups. It sucks for premades that already ran dual bard or dual blackmage, but I can see WHY SE wanted more variance in the parties. Bringing only 1 melee sucks, there should be incentive to have both Monk and Dragoon in a party. That's just my thought on it though.

I do feel, however, that instead of NERFING Limit Break guage, they should INCREASE the Full Party stat bonuses a group gets if it's completely varied. Boost how much STR, DEX, MND, INT, and PIE a group gets if everyone's a different class. Bam. Not only are you not screwed in Twintania limit break building, and not only do you have classes that can completely support one another, but your stats are increased. I'm not saying it has to be a huge increase, but any increase is a good increase.