It's amusing how you and alexhutcheson both corrected me in opposite directions.

For reasons Alex explains, interrupt handlers have to be really minimal. So you often use them to set state that makes the main loop branch off into the "real" handler.

But even then, you often don't want to wait a long time in that handler either. That too becomes a short-running event handler (though not necessarily a callback). Just not as extremely short running as it would have to be if did it in the interrupt handler.