Within the ASF, the use of the development mailing list is *the* method of development discussion. That's the reason for it.
Wikis are good for "after the fact" documentation.
IRC is good when a small subset of developers need to get together quickly to talk about some aspects of development, but it should quickly and completely migrate to email after the "pressing" matters have been dealt with. Same with thinks like "meetings over beer" and stuff like that. The reason, of course, should be obvious: it excludes by its very nature other developers. And you can't have collaborative development when that happens.
Also, in-the-open development via Email makes it easy to prevent such claims as "back door" activity. How can it be back door when it's openly discussed in the primary development scheme?
In general, however, such things as "we discussed this on IRC and we decided to do this and we'll post a summary on Email when we can" is never a good idea, and can result in kindly words that "development is always done on the mailing list" to fiery words that "people are trying to have their cake and eat it too by riding on the ASF name without adhering to its standard practices."
This is an issue that every ASF project has had to deal with in one way or another.
