This is, of course, a question which all Beatles fans have mulled over, and, until the opening up of the secrets of the recordings in the 90's, it wasn't easily resolved.
However, anyone acquainted with George Martin's utterances and with the Anthology series will now accept that Martin himself has no illusions about the genius for melody displayed by the group. Yes, they would have made it without him, but the recordings and arrangements would have been less brilliant.