It takes up a time slot, and if the time slot is 0 minutes, it still clutters my calendar space. The information is there, my calendar is not encumbered. But the information is very valuable. Please review them for more context. If Calendaring and Scheduling applications are to be interoperable across currently existing and future apps, then the message types they manage need a standard for movement.
Several applications support standalone journal entries.
In some products, like GroupWise and the host of it's predecessors these items are called notes, and can be sent to another users system. What is sent is not a mail message, because it only displays on the given day. It isn't an appointment, there is no time specification, only a day or date range. It isn't a to-do, since there is no completed status. Some of the notes that display might have originated with me, as in the case of a journal entry, and some of them may have originated from someone else.