Honestly I think that's the only time you really need to. Unless you're doing some kind of crazy racing stunts...
Some people say you need both hands to react fast and turn the wheel if you're about to crash, but physics usually dictates that if you have to do a two handed crank on the wheel then you're probably fucked already.
I've counter steered on ice before with one hand. If you're sliding or need to dodge something to the point that two hands will actually make a difference, or need to get the vehicle back under control, the timing is so critical that chances are you're already screwed before you even react. Jerking on the wheel is a good way to lose control though.
Anyway. Two hands lets you hold on better when you crash. That's about it.
Edit: if you're driving a semi or a bus or something though, by all means use two hands.