I first saw calendar spam in Yahoo calendar in 2008. This at least is an active opt-in which I have no problems with.
Going to be interesting to see if it catches on.
Sumatra Development leads the field of migrating entire calendar servers to Exchange. We migrate Oracle Calendar Server, Oracle Beehive, Apple iCalendar, and Zimbra to Microsoft Exchange keeping all meeting information and resource bookings intact. We migrate calendars server-to-server between Exchange and Office 365 while keeping meetings live and doing incremental syncs quicker than any other solution.