There's not a ton of use for the calendar since forum dated events aren't a major thing that tends to happen here. However, as White noted, the primary use case is to list birthdays of forum members - Something I, too, tend to check from time to time.
That said, I feel with the number of older inactive members and all the birthday list might be getting a little too bloated. I'm seeing members that haven't been active for like 8+ years, for example. Perhaps it might be fair to put a limitation that it would list only the birthdays of members that have been active in the last year or 2, as a means to filter the list from those users?