Yeah, the only thing you can really use fellows for right now other than lolbookburnz is VNM trials for Empyrean weapons, and even that's questionable.

Allowing them Abyssea access wouldn't be overpowered at all, provided they didn't benefit from Atma/Cruor Buffs. The vast majority of meaningful NMs would one or two-shot them with AoEs anyway. Not letting them into sky is just super weird, considering they're allowed in sea, as far as I'm aware.