Because, with a gamepad, i optimise position of shorcuts. I place them by theme and utility, and this is what I change when new (and more useful) skill comes up.
As a WHM, i won't use Earth I as often when I have Earth II.
As a BLM, Fire III will be place near Fire because I use them often together, Ice won't be used anymore when i have Ice III. I won't use Thunder I when I have thunder II etc... So i move them on a secondary hotbar. Etc...

And there's more. It really depend of the class/Jobs.

I'm only lvl 30 with my paladin, but i have no problem with this class at this moment...
And only change one thing for invoker.