Calendars have a built in Add Event form, which is fine except it doesn't have the ability to add form rules like any other Add Record form I'd create. This makes it impossible to create a consistent interface that includes calendars. What if, in other Add Record views relating to the same object, I have a field that gets auto-updated using a record rule? And what if that's a critical field? If somebody adds a record from a calendar view, the update can't happen.
I consider this a design flaw more than a feature request. Should really be fixed.