At a few places I've worked I had reasonable success with a similar approach by simply setting up some mail forwarding rules and MHonArc[1] to create customized internal web-based mailing lists capturing topic-specific company knowledge.
I think that with GrexIt, the value bring is:
- you can add a discussion to grexit with a button press.
- we replicate the discussion before it was added to grexit accurately in grexit along with all attachments
- we track follow-up emails on a discussion added to grexit automatically.
- inside grexit you can organize the content using shared label which are like email.
- Google apps users can just use their Google accounts to access grexit
- we are coming out with group based access controls soon which will let you control who sees what.