Think of your Email Inbox as a heavily fortified walled garden. Not mentioning the difficulties many have accessing their Email Inbox outside the corporate firewall, the Email Inbox contains a hodgepodge of business, personal and private information that most people do not want to share with others.
For many folks, the Email Inbox contains their most intimate secrets all mashed together into a single location: business correspondences, contracts, proposals, reminders, tasks, love letters, indiscreet online purchases, dirty jokes, pictures of your spouse (and kids), time-wasting games, inappropriate messages from co-workers and friends and lets not forget spam.
A number of the commenters to the post object that email is not intended as acollaboration tool, but I think that misses the point. Because email is pervasive, it tends to become the hammer for every job, no matter how ill-suited for the task. Furthermore, which vendor of email and personal productivity tools isn't calling them "collaboration tools" today.
As I've argued previously, Wikis and other lightweight Web 2.0 innovations notwithstanding, most of these so-called collaboration tools that people actually use today are far more about the individual than the group.