NEC Articles 350.10 and 356.10 both use the language "Where conditions...require flexibility or protection of liquids..."

In common practice, is this interpreted as only device which need to move or vibrate are allowed to be connected with flexible conduit, instead of rigid or thin-walled conduit?

Or is it allowed more liberally, that saving labor and difficulty is a good enough justification to use flexible conduit to connect devices?

Thanks for any insight,