Yeah, though there are additional benefits for purely solo designed duties for MSQ relevancy, beyond what I mentioned above.
1) The ability to use cutscenes during said duty. SE learned their lesson about with with Praetorium and Castrum Meridianum what happens when they combine cutscenes and Duties. I.e. First timers want to watch the cutscenes, but people whom are just here for the roulette bonus want to skip everything and nuke all the enemies and get out asap. With purely solo designed duties from the outset, they have all the freedom in the world to put cutscenes into the MSQ required duty because there will be no people rushing ahead while you're sat watching the cutscenes. Meaning that when people are doing the group version, no-one has any cutscenes left to watch, so everyone can just get on with the thing.
2) Increased ability to put in books to read. Since, in a solo duty you can go at your own pace, you can stop and read the books. I know that some of the newer dungeons have some books in that I'd like to read, unfortunately, PuG's want to just blast through at 3000 MPH.
3) More room to put in optional paths. One of the main points against optional paths is that no-one uses them, because in DF content, people just want to rush to the end to get their Exp/Tomes. But with solo content, people are more able to partake in optional side paths and little puzzles (Such as the Lost Temple of Qarn, before it got nerfed to better facilitate DF zergs)
4) Potential for additional objectives. Instead of just having "Hallway of Trash" > "Boss" > "Hallway of Trash" > "Boss" > "Hallway of Trash" > "Boss" copied and pasted for literally every. Single. Duty. They could put in additional things to do. "Go disable the security of this Garlean base". "Optional: Destroy the armoury to prevent reinforcements". "Hidden: Find the top secret lab > Optional: Find the research notes > Defeat the head Researcher!". Since again, they don't need to create content that has to only facilitate zerging through because 99.99% of people are only playing it because Roulette rewards...
Like, we go visit so many places in the course of MSQ duties, but then most of the time we're just trying to keep up with the PuG we're with who've already done it 50 times despite it only being released 5 minutes prior...
Trusts/Squadrons help somewhat, but there's still that particular issue with regards to immersion, when we're referenced or shown to be alone. I.e. A fairly recent but notable example is Mt Gulg and the trial at the end. We head off to the Trial, alone. Then *Poof* 7 people turn up to help us smack down the boss and then *Poof* they're all gone and we're alone in the cutscene where everyone else catches up to us.
There's been a few if I recall correctly.
I believe there was one way back in 1.0/2.0 and is the reason that there's a 2.5s GCD (As opposed to the industry "Standard" 2.0s or most modern MMO's 1.5-1.0s).
There have since been others, though I can't date them exactly.