Here's what happens:
Someone wants to do two things at once.
Aha, I can do that by making one thing an interrupt.
Oh, but wait a minute, nothing works.
I find out it's because interrupts are disabled during an interrupt service routine.
I realize (or I'm told) I can set a flag in the interrupt and poll it in the main program.
I then realize that the polling is as slow as the loop() is.
I then realize I could have just polled the switch in the first place. 