Do you know that scenario: My wife would like to fix a meeting and should have access to my calendar. I am not available, therefore she cannot just call me but - again - she should see my availability. Not uncommon, isn't it?
A typical solution for this: I have to run two calendars, one for business purposes and one for private purposes. As I am lazy by nature, the private calendar is always outdated, causing serious complaints from my wife. How do you handle this scenario: Well, a lot of people use online calendars to sync their Outlook with and give their family access to this calendar. A scenario that keeps the Security Officers in companies awake at night: Sensitive data is leaving the company - to fulfill a valid request by the users.
Now, McKinsey just had such an incident with Google Calendar (not that this is a Google problem, this is an industry challenge): http://www.infoworld.com/article/07/04/17/HNgooglecalendardata_1.html