As you are probably aware, outlook is very tied to exchange and sharepoint servers/services. I think that by making an extension available that interacts with Thunderbird in a similar fashion opens up a world of possibilites.
It is platform independent, so it would work on Windows/Mac/Liinux. Would give users something semi-familiar, which would help in the transition. etc....
Example:
There is a Zindrus plugin to sync google contacts with thunderbird. A funambol plugin to sync contacts. And a bi-directional google calendar sync for the lightning calendar extension. These features, on a non-platform specific environment. Something I have yet to see from similar open source projects like Zimbra or Open-Exchange.