Things like the calendar are particularly fiercely debated. There are many communities that do not use one. There are some that do - and invariably the ones that do, also rely on it quite heavily.
Breaking it out of the core has consequences for them. More importantly, it has all kinds of other consequences. It means you absolutely ensure your modding functionality is up to scratch. It has performance consequences. And probably most compelling of all, you can add to its functionality separately from the main core, meaning you can turn around updates to it separately of the forum - adding in functionality that users want without waiting for another major release of SMF.
On the other hand it also encourages doing things previously unheard of. Right now the calendar depends on SMF's posting routine; adding a new calendar entry with a topic ties them together and if so doing, it means every time you view a topic you end up looking for whether or not there's a related calendar entry. This is a non-trivial performance problem.
If you were to decouple calendar events from main posting, you would avoid that performance hit while still being able to do what you want to do with calendar stuff.
