Well, to be clear I said I DIDN'T use all 48, but that it's still hard to fit them on LOGICALLY.

However, I don't ignore abilities just because I don't use them that often. All 10 role skills are on my hotbar on healer jobs. For something like Break, you might ask why? That's because it is used in some ARR content (looking at you, Twintania), and also because my gameplay style takes me outside of the norms. I don't want Break to be in a different spot for every single healer I use, though. Same for other skills. I map my hotbars logically across all jobs. R2 is offensive. L2 is defensive. It's not a hard and fast rule, but in general I have a system that makes sense to me. However, as I run out of space, I start have less options. Maybe I have 6 slots left, and I need to place a new primary spell (GCD only). I only have spots available on my long-winded spell slots. So now I have a random GCD spell in the middle of my oGCD cooldowns.

I also have slots assigned for items you might consider trivial, because of unconventional gameplay. My L2->R2 WXHB is reserved for potions. Or it's supposed to be. But on some jobs, I've had to put random abilities on it. The result is a lot of confusion when switching jobs. I'll try to pop an antidote on PLD because on my other jobs, it's in that slot. But I end up wasting Tempered Will.