well, one of the largest disadvantages of OpenOffice compared to MS Office has always been the lack of an Outlook-like e-mail and calendaring application. With Lightning (which is supposedly going to be part of Thunderbird by default in the not too distant future) Thunderbird is somewhat closer to Outlook's functionality. Perhaps this is why Sun became involved in the project - so that eventually OpenOffice can include a credible alternative to Outlook as well.
In my opinion Thunderbird will not be taken seriously by most large organisations until there is also an Exchange-like server offering which works with Thunderbird.
Since it seems that Thunderbird will simply become part of the OpenOffice suite (while remaining a separate project), I guess BirdieSync will remain an available add-on regardless whether you installed Thunderbird/Lightning separately or as part of OpenOffice.