A tradition of performance is an important element for several reasons. On a bland methodological level its an important variable. More seriously, it dictates so many things; the style, format, length, even the genre of the “text”. The role, and expectations, of both audience and patron and so on. It really is important. we have to try and establish something about the context from which these poems sprung. I agree that traditions don’t write poems; but they dictate the type of poems being written, how and when they circulate. The problem is the epic tradition is the one we know the least about, frustratingly, we’ll always know more about Stesikhoros or Pindar than a Homer or even a Peisandros.
“Many scholars seem to assume that the Iliad was first created and only afterwards written down, but what on earth does that mean? Hasn’t it been showed that in an oral tradition, every performance is different…ktl”
Well technically in a living tradition even an exact reproduction would be “different” due to slight differences in intonation etc since its a performance. But, no not necessarily, I think the rubric of “ever changing” as established by Parry and Lord has long been shoved aside. I mean we know of traditions where exact reproduction is imperative, like with the Vedas. Actually this kind of subservience to a notional poet/group of poems would be just the kind of situation where written copies might be handy. In that sense the earliest texts might have belonged to groups like the Homeridai or the Kreophylidai, as attempts to control the text. Again though, there is a lack of evidence and it all depends on how productive you think the tradition is around the 6th century or so.
I do think it possible that the creation of the poem and the writing occured at different times though.
“Anyway, oral dictation theory just doesn’t explain the kind of textual problems I mentioned earlier.”
Well, again, how do we imagine the earliest text(s) to be used? Say we have an archetype, we can roughly guess that long/short vowels a, e, o weren’t differentiated, no breathings, boustrophedon etc were attributes. Say it was taken down as via dictation, as other copies are made and circulated, other versions are sung, then of course the kind of problems like you highlight would be introduced. Perhaps enough to spur editorial activity like that of Zenodotos et al. Even if the poet wrote it out himself, he didn’t make all subsequent copies and mistakes are bound to occur.
“Yes, but I think they say those expanded songs are quite awful in quality, not at all like our carefully thought out Iliad.”
No, this is subjective. I mean we possess a certain aesthetic, itself heavily influenced by the Odyssey. Highly wrought literature can exist along different lines, look at the Mahabharata. Actually quite a lot of the dictated texts from India and Crete are pretty damn good. I basically reject the “argument from quality” as inherently flawed and defer to the oralists on this one.
“Isn’t West’s idea just so much more elegant …ktl”
I think once we free ourselves from the earlier baggage of the oralist position, that a poet can have nothing to do with writing, that all poems are inherently alluvial, that they must represent single time events then we realise it doesn’t have to be either/or. We need to entertain the posibility of oral poets who can deliberate, change and alter. God knows there is enough ethnographic evidence. My favourite being the Irish filid spending time in dark rooms to contemplate their songs. I just don’t necessarily think this figure is also the writer. Possible, I just don’t see the need for it.
“I’m continuing with this, because the Homeric question is one of the driving forces of my life…”
Dw, I literally have the same obsession. It is not a shameful one. 
I know I’ll never have a definitive answer, on the other hand I just wish I could for once feel safe making a statement without a dozen citations and bars and charts backing me up. I know, I know, it goes with the territory. Sometimes, I really wish I had just ended up pursuing tragedy instead…but it doesn’t have the same lure does it? It doesn’t have the same…incomparable… fervour to it!