I don't think this has been discussed/polled too much, and I think this thread is a good idea.
I think my play-time will be over 95% in Single Player mode, though I'll almost certainly use the Multi-Player mode at least to some extent.
I'll do a Single-Player playthrough, when I start the game for real. To me, BG3 is first and foremost a Single-Player game, as are most "long games". I might do one or more further playthroughs if, by the time the game is released (and definitive editioned and modded), the major User Interface obstacles between me and the actual game have been removed or alleviated enough.
Since Multi-Player is a possibility and I have friends as interested in CRPGs/DnD/Baldur's Gate as me, I'll probably try some MP play with them. But I doubt we'll have the regularity to play a full playthrough together.
(Also : I only ever played BG 1-2 in Single-Player mode, though I never "solo'ed" them but always had a max-size party.)
And for what it's worth, I too would be disappointed if some aspects of the game are subpar, especially when playing SP, because "MP play has to be possible" or "it isn't that bad in MP mode". (I'm thinking primarily about party controls, as well as the Continuous Time Mode vs Turn-Based issues).
