At around mid 30s each job becomes more and more distinct. By the time people reach 40, the jobs are so specific that it becomes difficult for certain jobs to do to certain things.
For example, up to a certain point, a WHM can be replaced easily by RDM. But after 40, RDMs become professional enfeeblers, and WHMs enfeebling skills become much harder to level, yet we learn more MP efficient spells to keep a PT going. (Regen II, III, Stona, Viruna, higher level Protectra/Shellra, Cure V, and big Curagas to bring back a bomb damaged PT) So eventaully, it becomes difficult for either jobs to do both at once. (While there are some odd RDMs that enjoy doing everything at once :))
Same with DD jobs... A WARs DEF won't compare to PLDs DEF, and hence even as a blink tank, WARs will have difficulty tanking, probably up to level 75, when they get Utsusemi 2. PLDs Attack, on the other hand will level less, and will become impossible for them to become serious damage dealers, until, again, they reach 72, finish the final WSNM, and get Savage Blade.
Similarly, Mages will use more MP per kill keeping every body healthy, Mobs properly nuked with big nukes, and keeping everybody buffed and debuffed. Hence, the need for "refreshers" such as a BRD or a RDM.
It becomes more difficult for one job to perform optimally without the other. I guess you can call it more dependency will become necessary.
So you see, as the mobs get higher in level, more specific jobs becomes necessary for efficient EXP session. While there are times when strange job combinations will work (BLM, BLM, BLM, BRD, NIN, THF for Mana burn PT or MNK, MNK, MNK, WHM, RDM, PLD for a "Bones" PT, RNG, RNG, RNG, BLM, NIN, BRD for scary fast EXP) these situations are unique and rather rare. However, please note that there is exception to all, and I'm sure there are gifted players with enough player skills and cash for gear enhanced traits to play two job roles on one job.
Fightings Ts and VT EXP become almost laughable post 50, since the EXP per kill will be less than 100, and even with chain 5, it won't hit up more than 150. Combined with shortages of certain job traits in a non-optimal PT (missing tank, lack of refreshers, no healers, no nukers) the mobs will become harder to kill and down time substantial, which ends up killing a chance for higer chains.
Believe me, I have been in PTs that tried...chugging Yagudo Drinks and Choco milks in a PT without a refresher, fighting T and VT Dahlmels and Rabits with occasional hobgoblins in Bibiki around 68ish. >< (PLD, THF, SMN, BLM, WHM, DRG)
...so...as much as we all hate, we have to sit around and wait. Most of my friends (and myself included) fish while LFGing and it helps pass time and make chunk change on the side.
Finally I agree that waiting for 30 minutes or more for a replacement to log on, convinced to PT, and arrive at camp is a big waste of time. As you start EXPing in god forsaken zones like Valley of Sorrows where a replacement can take up to an hour to appear, you should try to convince your PT to agree to a set time limit, and all disband and reform. ...at least you'll get to fish again in Port Jeuno while you wait...