I'll main PLD and use the capped tomes on that class first. But I also keep at least a DPS (DRG) and a healer (WHM) with gear up to uncapped tomes level, just in case I end up in a group that needs something else (and I actually play the 24-man raids on DRG more often than I do on PLD). That's the extent of my multi-classing.

I had all jobs at 50 from ARR to Heavensward (leveling ROG/NIN and ACN/SMN/SCH along the way) and ended up NEVER using some of them (BLM, NIN once it got leveled, for instance), whereas unlike the crafting and gathering classes, they all had a use, whatever limited it was (repairing my own gear on the go, melding for myself or other people, etc).

Therefore, I decided that from Heavensward and on, I'd level all crafting and gathering classes, but only would keep my main and a class from each other role.