Trusts are great during the msq. I would often use them for my first runs to get a feel for boss mechanics before doing them with a party later to grind exp or get gear.

Once you finish the msq it all goes to crap. All the characters are set to 71 and it takes six runs of the first dungeon just to get them to 72. And everyone levels individually, so if you start as a dps but then want to use trusts to level a healer, guess what you gotta levle up a second dps in the trusts too.

I don't think I'll be leveling with trusts for alt jobs.