Since starting this thread, I can see that some of the replies seem to feel that it's a debate as to whether Marr deserves any credit at all for the Smiths, and some perhaps can't get their head around exactly what I'm suggesting (though others obviously can).
One point - there's absolutely no ambiguity about the fact that Morrissey created the vocal melodies, which were entirely improvised by himself over the backing tracks - until Morrissey went into the studio and laid down the vocal, the rest of the band (Marr included) had no idea what the vocal melody was going to sound like. We know this because Marr himself described the process, citing in particular the example of 'I Know It's Over' when he was blown away by the strength and beauty of Morrissey's vocal melody.
Another point - the nature of the songwriting credit was not self-evident to Morrissey and Marr: that is to say, on the first album, songs are credited as 'Words by Morrissey, Music by Marr'; on subsequent records this has changed to 'Songs by Morrissey/Marr'. Somebody obviously felt that the original version of the credit wasn't an adequate description of the songwriting process - wonder who that could have been? My point is that I don't feel that the replacement credit - 'Songs by Morrissey/Marr' - is also misrepresentative, suggesting as it does a straight 50/50 contribution to the creation of the songs.
To my mind, a better credit would be 'Songs by Morrissey; Music by Johnny Marr'. Marr created basic backing tracks (a rhythmic chord progression and perhaps a riff or basic harmonies). I know that's an unusual credit, but theirs was an unusual working method. He gave these to Morrissey, who then effectively created the songs, as most people understand a song - i.e. the vocal melody, i.e. the tune that you hum, the words that you sing. Following this, Marr - and the others - were then able to embellish the recording with further layers of harmonies, using the vocal melody as their guide. This is what would normally be referred to on other artists' records as 'musical arrangement' but which in Marr's case seems - along with the original basic backing track - to constitute his claim for a songwriting co-credit, although the musical/arrangement contributions of Rourke and Porter were not considered to be grounds for such a claim.
So that's my argument really: the music was Marr's (and Rourke's, and sometimes even Porter's too, for that matter); but the songs were Morrissey's. This doesn't mean to say that Marr has no input into The Smiths, or that he doesn't deserve any credit. I just don't think he has a valid claim to songwriting credits, at least no more so than Rourke and Porter.
S.F.