You say this like it's freakin' easy. "Oh, yeah...they just need to do that". You're missing a few things:
1. Devs always have a well-defined idea of what they want a class/job to do. *This is software design 101. Figure out what everything does!*
2. You cannot always predict what players are going to do with the dev design. *You cannot test everything.*
3. The easiest way to conform to the original dev design without breaking the game completely is to make tweaks to certain abilities. *If people don't like the original dev design, there are other class/job options.*
Everyone always complains and says, "The devs have no idea what they're doing". Actually, they do. FFXIV is definitely one of the better games in terms of job design and balance. Each job has an intended role that it fits very well, on the whole. Yes, sometimes a job suffers here or there, but the adjustments made usually make up for that -- WHILE still allowing the job to conform to the intended design.
The problem is, everyone wants to sit there and have it their way. Well, sorry...YOUR WAY isn't really what the devs are going to do. (If you're lucky, they'll do it "close" to your way.) If you always want it your way in a game, then go develop your own dang game!