Sort of agreed even though high level Experts will destroy your team if you just auto (let's say this happens after level 90) therefore the biggest advantage that Experts have in relation with Ultimates is the fact that even if said Expert is level 126 (as it's currently in my Union for the day) or even higher, if you use your skills wisely you can go a whole battle without getting touch by a raid skill, that dismissing that skills like Amaterasu's will make you potentially take zero damage around 50% of the time.
Saying that the amount of experience that you'll get it's humongous in relation to what most people can solo (regarding Ultimate that is really tough after 100 even if you're there manually doing it of course) and it only takes around 5 minutes.