You are right when you say marketing is important and some very good writers are very bad at it. But there are people that could do both too. There are other books that need artists(text books of history, biology...) and other support people too.

When I say there is no need for publishers I mean the hard logistics and financial experts that today are needed. Other people will be needed, like web store experts, but you don't need to have a minimum of 20000 readers to print like today.

Today, like yesterday CD labels, publishers hold the rights because they have access to the physical market and you can't sell without then, they then get all the money and no cost in ebooks. This is going to change when writers don't need physical market because people buy digital.