I agree that web-based solutions could reach enough people for them to be a feasible replacement for email in terms of availability. The point I was trying to make by talking about access is that everyone has an email address I can send mail to, but generally access to project-specific tools are limited to folks on the project. So if I hash out an interesting technical approach with someone outside the project or outside the company, it will likely be done in email or via the phone.

Once I have resorted to using email, eventually I will forget to move a conversation over or I don't think it will be important and our "repository" has gotten out of sync, which leaves me with a merging issue (do I trust what my email says or the website), and eventually a duplication of effort (I know we talked about this, but my email says x, and the website says y)

As an aside, as I was writing this, I also realized that I tend to use email as my "archive of knowledge". When I move on to a new project I can search back through my email to find relevant discussions and apply them to my new work. I have not seen a tool with a similar ability to archive and take with me my lessons learned (whether because of company policy or lack of feature in the tool). I know lots of managers who religiously archive their email to avoid losing some obscure piece of information or process from years ago.

True, but people are already using email. Everyone knows how email works and how to attach files. Not everyone knows what a "writeboard" is and how it's different from a "message"