Crossposting from reddit as people seem to be glad to be made aware of this:
As you go through the story, you can use the trust npcs for fast queues, to spam dungeons solo for gear to gear up, for desynth, for xp, whatever, even till the 1st lv 80 dungeon. (half the loot but 1/4 the people to lot)
Also one thing I've seen is the npcs are programmed to do mechanics properly so they're also a great way to learn a dungeon.
But after the lv 80 trial and turning in the Shadowbringers quest, you get a message saying you can "now level up your trust npcs"...which is a nice way of saying they get nerfed to lv 71 and stopped from accessing later dungeons till you level them up.
...which starts at 2.5m per run in Holminster, and this is the only xp they get; nothing extra per mob. Getting them to 72 is 13,881,000 xp so they'll hit 72 after your 6th run. A partial run gives them nothing.
The npcs you leave behind also gain nothing, so pick wisely. My recommendations (playing as dps): Alphinaud as "sch", his shields can save you if you mess anything up, and Minifilia does a 20-second trick attack pretty frequently. If you don't go dps probably Y'shtola 2nd dps, for me Alisaie liked to lb as rdm on boss pulls, which is kind of a waste. Thancred's the only tank option at this point, as the npc list no longer changes per dungeon. (other healer option is Urianger)
Overall the trusts are great for queues, loot, and learning mechanics, but they are pretty slow at dps. On the other hand, they never complained about taking down trash mobs themselves when I had to afk a moment.
And obviously you'll need to accept the nerf to carry on in the lv 80 content; I just felt some people might want to get some use of their fully-leveled pals before that. So just be aware of this if you plan to use them in later dungeons for whatever reason suits you.
I think it was a bad decision on SE's part; if not completely scrapping this extra grind it'd be nice if the dungeons you use them on during MSQ would count to their xp. Levelling them up is understandable... it's just a real feel-bad to have their levels taken away with no warning, but at least knowing in advance you can make a decision on how much to spam before moving on in the content.