The question you should be asking is - which shared calendar protocol should we choose?
Good call on the question remark, I'd disagree with your answer.
The problem is that iCalendar isn't calendar 'line' or 'sharing protocol, it's more of a 'serialization/persistance' protocol. iCalendar does not define any connection or query methods. Things like that have to be defined if there is to be any interop. We've actually written tools around the iCalendar/WebDAV combo, they work great for smaller teams, but you run into problems very quickly has the team grows or the calendar's use increases.
As things settle down, CalDAV, a.k.a RFC 4791 will probably become more of an entrenched calendar sharing standard. I've been working on a CalDAV Outlook plugin, Open Connector for quite some time. CalDAV is supported by Apple Calendaring products, Mozill thunderbird, Oracle calendaring server and a bunch of other open-source and commercial packages.