Basically I swap equip on every job I have, it's an easy way to improve your play in pts without improving your skill as a player (if that makes any sense). I've found over time that certain swaps are essential and others non-essential. (IMHO)
RNG: Swaps for acc / dmg / str and mnd. Swaps for acc are relatively self-explanatory: if you miss Sidewinder/slugshot you need more acc, swaps for dmg / str are not the same thing, I swap STR rings to built tp and run towards the mob if I'm in a 'tp burn' pt, and DMG gear to do more dmg per shot if my ranged acc is not a concern. The nice little semi-secret swap I do on RNG is MND gear, which GREATLY increases the effects of all status bolts, yes this means that with a ton of +MND gear you can take that +6 add effect of holy bolts and make it 50. I'm not kidding, it's that high. Being taru with low mind it's night / day if I take on/off my mnd gear when using the Obow, elvaan (maybe even hume) might not see this huge diff, but I did. +25 MND with swap in, and add effect ranged from 30-55 on weapons in sky. And of course macro in the AF for the JA it enhances.
BRD: Oh the equip nightmare job. "To use all +1 instruments or not to use all +1 instruments, that is teh question." Well for 69 levels I was a 'purist' using the +1 instrument (Or rare equivalent) for every song. Then one day I needed to 'hot swap' in a pt from RNG to BRD and didn't have time to grab all my gear. What I found made me /cry after all the lost things I had from having full inv. space. Basically the only swaps you 'need' on BRD are elemental staves for songs you cast on a mob, and the +1 for those songs. The reason I say this, is that the +time added even with +2 song is so low, it makes no difference to a good brd anyway. (If you're the kind of brd that lets your songs go off of pt maybe this helps you) but for me I have a sequence I do when I pull sing melee songs, sing mage songs, rinse repeat. So the +song was doing nothing for me. Elemental staves with threnody are great if you are a threnody addict (which i am) and earth staff for elegy makes it near 100% stick if you have even decent +CHR.
THF: Pretty straight forward, acc gear for TP build, balance of AGI STR and DEX for WS. Invauable, as anything other than mithra I'd guess. As hume this kind of swapping make me 'mithra-like' at lower lvls. Being able to build tp as quick as any other job (Besides a good SAM or RNG) and doing very high dmg WS and SATAs.
DRK: Same kinda thing with acc and dmg for tp build and STR for WS, but it also depends on what food you use. My only little secret for DRK was using INT up items for dark magic spells, this helps a ton if you don't happen to be taru, and if you are basically makes it a sure thing.
BST: The swapmaster. Lvling bst from 1-57 I had a set for acc, def, charm, +MP, and ws. And even a pallate of blend macros if none of those went well. This job is where I saw the beauty of swapping become utter magic, with acc gear and STR WS I could raise my xp per hour dramatically, and with +charm / CHR stuff I nearly never failed charm, and the charm lasted a very very long time. (I happen to be one of those unlucky critcal fail bsts, no equip can fix that. lol) Being alone most of the time, or in small bst pts the wealth of equip you have can easily mean the difference between life / death on a minute to minute basis. If I had to pick just 2 swaps it would be tp and charm, if I were limited on space.
RDM/WHM: Yes the staves / MND / INT gear for debuffs. If you play these jobs hopefully you understand the massive importance if keeping a mob debuffed. What I did notice is if I debuffed with a very high MND with WHM debuffs as RDM or WHM, not only did it stick more often, but the effect of paralyze seemed to happen much more often. I'm not sure if this is confirmed but it sure helped me in some not so hot pts where paralyze going off alot and slow sticking meant the difference between chain #3 or no chain at all.
WAR: DEF STR TP and Emnity for warcry / provoke if you are having trouble pulling hate off a very good tank during a switch-hate SC. AF for the emnity if you don't wanna buy rings / earrings.
PLD: VIT CHR/emnity and +MP. CHR / Emnity to voke / flash, VIT to tank and MP to macro in for start of fight and macro out with the excess mp is used. The +CHR / emnity is a very helpful thing if you have dumb BLM or trigger happy melee in your pt. The +MP may only give you an extra flash and Cure II but, as PLD you know that might mean life or death. Also obviously the AF gear that enhances the JAs you have.
BLM: INT +elemental skill and +magic attk bonus. I'm not an expert BLM, but as taru what I've noticed lvling to 61 is that a good blend of these helps, not really maxxing out one unless you fight IT250 base mobs, which are rarely good xp per hour except in those nearly-non-existant super-pts. Personally I only macro'd in the staves for the element of the spell I was casting as BLM, left the other gear static. Dmg increase for each staff and the reduction in spell resistance is A-MAZ-ING. Get 'em.
NIN: My personal fav. macro-fest. INT for Ninjutsu, acc for tp, def for when shadows go down, str / attk for WS, evasion for hard / higher mobs, attk for lower / easier stuff, emnity in heavy dd pts, the list goes on. The biggest advantages in all these combos I've seen is the +dmg on elemental Ni's when using full +INT, 120-130 vs. 80. You do the math, lol. +emnity helps alot, and I think that it's the best swap a NIN can have. +evasion seems to help alot vs. VT and barely IT. IT++ seems to make no difference whatsoever. Attk / STR for WS is invaluable b/c without it, NIN WS can be weak as hell, but with it we can do decent WSs. VIT and DEF is a personal preference thing, we have such awful VIT / DEF to start with it's not a huge jump but everything helps. If nothing else make a macro for taking out those snipers when you're getting pounded. Maybe you'll live to see another mob.^^
Other jobs upon request, these are just my fav. swap-jobs.