Well, let's see ... let's see ...
Certainly Yesterday is not the same as any other song ... (almost released as a Macca solo song)
She's Leaving Home's story, for example, is quite diferent, because the idea of the arrangement -and the arrengement itself- was entirely Paul ... he needed Sir George just to write -not compose- the notes on a paper: that's all ... Sir George couldn't and it was done by Mike, OK ... Mike didn't compose anything on that one, so there's no place for credits, as far as I can see ... (oh!, John sang backing vocals too)
In the case of other songs (She Loves You ...) if the arrangement, harmonies and other things were defined by the composers (John+Paul, John or Paul) they matter would be a diferent one -the paper of the producer wouldn't be as important as it's in Yesterday- Sir George would collaborate doing his job as a part of a team, that's all, ... but his role in Yesterday is prominent, he's not just interpreting their ideas and if John got credits he should have too, why not? ... it would have been fair acording to the facts ...
If we think that Paul wrote the words and CHORDS but not the music - Martin scored and arranged it for a string quartet. Certainly Sir George defined the song, gave it its personality ... He did more than John, didn't he? ... What did John? ... but who got the credits instead?
It's a Macca song ... or a Macca/Martin (The Sirs) song ... but it's not a Lennon/McCartney one (John said once that he did nothing) :
It seems that everyone (DJs, public, ... ) have done more for the song than John
... but he got some money ... very unfair :-/
Summing up: the matter here is that John got credits ... if the line would say 'by Paul McCartney' I'm sure that nobody would be talking about all this ...
P.S. I think that the only ones that were just doing their job were the classical musicians ...