I'd had all these same thoughts. currently the grind isn't worth the reward, and to be used effectively would have to spread across at least 6 classes (2 of each role) since your character classes lvl faster than the trust characters do. I mean its 13 dungeon runs to even reach the second dungeon, and that 13 has to be done again if you switch role.
Having the crystal Exarch available might be nice too cause it would allow for one multi role character to prevent issues.
I agree 100% with the original post up above, and just to add to it I came to the forums today with a mind to suggest any one of three options:
1: implement a way to lvl trusts externally like squadrons or retainers etc. OR
2: reduce the experience needed to be more reasonable so trusts are less of a repetitive grind and locked so severely to a single role OR
3: create a better reward for the trust system, whether it be some kind of voucher that you obtain as a reward while lvling, or some new cosmetics unlocked for completion or a mount; doesn't matter. but the current reward does not justify the current arduous grind trusts are in their current state.
I love the trust system My first time through with a new set of characters was wonderful I ran three trust dungeons on the way through the MSQ, and was excitedly anticipating using it to farm the lvl 73 dungeon after I cleared when i found out about the reset to lvl 71. I still again enjoyed my first run through of holminster, but that joy I felt in seeing the characters interact and react to the circumstances of each dungeon was lost when I realized id have to ruin the same dungeon at least 13 times in the same role with the same characters, wasting experience gain too as id be over leveled by that point, to even access the lvl 73 dungeon I was very demotivated. I ran it like 7 times and got my first characters lvled but since I had done a few runs as healer, and switched urianger and alphinaud once to see the differences i still needed to run it 3 more times to get my healer there as well.
Other systems with similar completion rewards also provide rewards during the process (scrips/exp for example from custom deliveries which require way less time and effort to complete as well)
Anyways, I still love the trust system, but the current form of restriction/reward it has is even less inviting than the worst portions of the eureka slog. (im one of those weird people that actually kinda enjoyed eureka at certain points)