Possibly I might be thinking a bit too deeply. A feeling that some songs/tunes might not be what they are without a performer other than the main artist.
Not saying that the “secondary” performer is unrecognised, just wondering what might have been.
Lou Reed - Walk On The Wild Side, Herbie Flowers on bass.
Gerry Rafferty - Baker Street, Raphael Ravenscroft on sax
Steeleye Span - To Know Him Is To Love Him, David Bowie on sax.
-lan Stewart - piano on ‘Boogie with Stu’ - LZ
-Iman Karniparinpil played jewsharp on ‘Kashmir’ -LZ
-Ian Stewart also played piano on ‘Rock and Roll’, LZ
-Viram Jasani played tabla on ‘Black Mountain Side’ - LZ
-Sandy Denny - Battle of Evermore - LZ
Leaving aside opera, that is how I feel about the vast majority of songs, though to me “main artist”, as you put it, is the singer I heard when first liking a song, not necessarily the singer who originally sang/recorded.
There are exceptions - e.g. Sid Vicious singing My way did it far better than Frank Sinatra.
(Opera is different, perhaps in the same way that classical music is different from non-classical, with multiple versions being the norm, and the overall performance being the focus.)
With instruments rather than vocals it seems easier for people to catch the original style if playing, but if they don’t get it right then the same applues for me as with vocals.
The most famous, or notorious, occasion that comes to mind is George Harrison’s appearance on ‘Badge’ on Cream’s ‘Goodbye’ album. For some reason - that now escapes me - George couldn’t be credited as co-writer of the song with his chum Eric Clapton (or playing on the track), so is named as ‘L’Angelo Mysterioso’ in the album credits.
(Their friendship foundered not long afterwards when EC had an affair with and then married GH’s wife, Pattie Boyd.)
Picking through this discussion, there would appear to be a possible explanation that fits the apparent contradiction: GH correctly identified as co-writer of the song but his contract with Apple (or whoever) preventing crediting as a player (as opposed to writer) on the album sleeve, so @Graham55 partially right but incorrect detail…
A random thought on the above, when Cream recorded Goodbye the Fabs had already set up Apple Records, wouldn’t George have been under contract to them, in which case why did they limit their own freedom?
Back to topic, regular readers of the WAYLT thread may know my views that there is a special magic when lead guitarist and vocalist are so sympatico that their interplay is like a vocal duet, and a couple of examples that immediately spring to mind:
Mick Ronson on Ziggy Stardust, without his guitar lines Bowie’s magic would have been nowhere near as powerful for me. (I don’t know if he’s undersung, but definitely welcome.)
Declan Sinnott’s guitar work on Mary Black’s albums, I’d especially call out Bright Blue Rose from Babes in the Woods where he plays one simple line at one point, which 32 years after I bought the album on release can still take my breath away.