I know, I agree. I just don't like seeing level 75 RDMs who put bare minimum effort in the job while I put so much into it. Then again, anyone that is leveling multiple jobs, is less able to put 100% in every job. One reason is the r/ex items that cater to certain jobs.
In some cases that doesn't matter much. If you're leveling RDM, you can use a lot of gear that you already own for WHM, BLM or SCH for the most part. I can definitely see where you're coming from if someone was doing RDM COR and WAR though.
3 jobs will absolutely never cause inventory problems. I have rdm, blm, brd, and rng fully geared (by fully geared i mean anything i'd ever need, including tank, fr, and melee gear for rdm, melee tp build for kcrng, max mp, 380 ss for brd, and some other gear sets that 99% of people don't have), as well as zerg gear for 72 drk (only 7-8 items that i didn't already have for other jobs), and a bunch of r/e gear from nin (10-15 items i think). While a ton of gear does indeed overlap between rdm, blm, and sort of brd, almost none does with rng, which has full tp set, /ra acc/damage sets, and slugshot acc/damage sets, and i could still squeeze in gear for another job (maybe
two if they were both mage jobs).
4 jobs is definitely the minimum before you'd ever start to run out of room, and most people could easily get 5-6, and possibly even 7-8 depending on the jobs, how well they geared them, and possibly how much they keep in dbox.