It all comes down to balance. Designers are humans, and they, too, have their limitations. Balance is much easier to pull off when you have control over the variables such as party set-up, size, possible skill set-ups.

Freedom is the complete opposite of balance. Freedom, to designers, means more variables, to the point where they are impossible to control. That only applies from a gameplay perspective though. Aesthetical freedom is only a strain on the art assets.

Freedom vs. Balance is what MMO devs struggle with. On the other hand you have the concept of an MMO, on the other hand you want to create a game with balanced=>tactical=>challenging=>meaningful gameplay. At the end of the day, either extreme is never the answer.

While I wouldn't want SE to go to such extremes as WoW does with it's search for Perfect Balance I want them to be close enough for XIV to feel like an actual game. For that to happen the designers need to be in control.