Considering that 37signals products are web-based, they also work natively on a pretty large percentage of available hardware and software platforms (at least the ones you're likely to be using).
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.