I think OP has a point here, that trusts are a good learning mechanic. Most of the time, the NPC's react early to mechanics, and that can be very helpful your first time through. Hell, with ShB dungeons, I was glad that I got to do it with NPC's who would do the mechanics, even if they forget how to DPS while doing mechanics. I personally think that is the benefit of playing with other players, they don't forget how their kit works when mechanics pop (for the most part). Personally, I'm kind of a speed freak, but like to pull at the pace of my healer, and Urineanger sometimes does forget to heal in an inorganic way that most player healers don't.

I do think doing a trust first that unlocks the dungeon for roulettes is an interesting idea. It would be more story oriented, as FF14 is a much more story focused MMO. It would also provide time for players to enjoy the cutscenes (looking at you tank and healer mains). One thing I've noticed is tanks/healers do get pressured in duty finder to skip cutscenes because "You can go back and watch them." This is inherently not how FF14 is designed, Yoshi P. has said numerous times that story is really the big component to 14. At least in the trust, you can skip the cutscenes without two dps yelling, "OMFG CUTSCENE WATCHER SO SLOW," which I've seen in a lot of situations. For a game whose biggest draw is it's story, this is the most logical way to tell the story you want to tell and let players have some choice in how fast they go through the dungeon on the first try. It would also help tanks/healers at least see mechanics and how they should be reacted to. I don't really give people grief if they mess up their first few times in a dungeon for that reason alone, not everyone can be lightning quick to understand mehanics, not everyone remembers that some mechanics are shared, not everyone knows that most mechanics are a combination of other, previous mechanics. The trust gives a way to see any new mechanics that may get people screamed at by losers for messing up their first time through the dungeon.

I dunno, I've tanked since 2.0 came out and I can certainly understand the want to immerse oneself in the story or have a low pressure scenario. Is it most efficient to do only trusts? No, definitely not. Is it more immersive? I would say yes.