It's the solution to having to choose between damage and enfeebles on BLM. Better cures or better Enhancement Magic on CNJ. Faster TP Gain + More Weaponskills vs. Higher auto-attack damage/less but slightly more powerful weaponskills on MNK...etc etc... Being able to swap on the fly efficiently in battle as a blm to go from dealing damage to needing to land a sleep/bind/heavy (guarenteed) to save parties ass from sure defeat... being able to make a stoneskin last a lot longer midfight to help mitigate pulling too much hate from cures while still being able to cure at full potential... there is a lot of things there. (Gear swapping may even allow some fights to be done with ONE WHM instead of the standard TWO.)
And of course I am sure you can come up with some whatever comment to make it seem like none of this matters. But the point is it matters to some people. Adding it to the game will not be saying "Do this or gtfo." Since obviously enough people don't like it.