With regard to the bassist role, there is a school of thought that BF's loyalty to Graham S. ran deep enough to want the role sufficiently semi-vacant, just in case Graham should be able to resolve his emotional problems and return to the fold. As BF's autobiography looks likely to remain unwritten, we'll probably never know...
Rik K. was possibly recruited primarily to fulfill live commitments which had been agreed before Graham's departure; or perhaps he just wasn't a good fit in some way. Thereafter, BF was prone to saying in interviews that this bassist or that one looked like being a keeper, but none of them ever stayed! Johnny G. was hugely influential, but didn't seem to want to tour; Sal did the tours, but was coming to the end of his visa (and would presumably have required a work permit); John W. wanted to be a vocalist too and King Crimson offered him that freedom.
As for musicianship, the title of "virtuoso" would probably sit most comfortably with Eddie, IMHO; his academic studies were shelved in favour of joining Curved Air almost straight from school (I find it easy to understand the lure of standing next to Sonja Kristina in her youthful prime!), but his violin and keyboard skills have never been surpassed in either Roxy or BF's bands. I'd be surprised if the original intro to SFE wasn't penned at least in part by him, while As The World Turns holds a special place in the hearts of many here.
Musically Colin Good was very important to the jazz & swing related music (I suspect he probably assisted with recruiting and rehearsing the jazz musicians and with scoring their parts), but possibly less so with the rock stuff, to which he is not naturally suited. His value to BF was probably as much as Musical Director as it was as a band member.
As MD he would have taken some of the load off BF's shoulders and reduced the need for, or risk of, micro-management. I'm sure it would have been among CG's duties to get the band members to rehearsals, soundchecks and gigs on time and, on occasions, to supervise those soundchecks. I witnessed this at first hand in Dubrovnik, partly because Mrs Smudge & I stayed at the same hotel as the band and partly because the gig was in the old city square with difficult acoustics, which led to the soundcheck being lengthy and public (for which reasons BF chose not to attend it and to entrust matters to CG).