Separate applications do not break that; you can still send that meeting request from the calendar app, or see availability when writing an email.

Just because they are separate application does not mean, that they do not operate on the same data. However I appreciate when I can use the full-blown GUI to look at calendars in another window, even when I'm in the middle of composing an email. The modal switcher in Outlook was one the worst thing I've experienced in productivity software.