Sometimes IK/FK switch is a necessary thing. For example, your character throws the stone (arms FK) and then put his hand over his lips in some emotional affect (arms IK) and then quicly walks away (arms FK). In this animation it is very convenient to use such switch. I won't like to do it all in just FK. So, in this…
IK will also work. Maybe it was not the best example. But sure as hell, there are times when you need this switch. And the last thing you want is a flip.