"Thunderbird's major shortcoming is the absence of an integrated calendar," the report says. A future release is expected to fill that void. Once calendaring becomes available, Thunderbird will be much more competitive, the report suggests.
No, Thunderbird's major shortcoming is that it's bad -- just a little bit harder to figure out and a little bit more time-consuming to use than other mail clients out there. In other words, the opposite of Firefox, which is significantly easier to use, faster and more powerful than Internet Explorer.
Adding a calendar to Thunderbird won't help.