I've just fitted cruise control, and wouldn't recomend not putting the correct switch on the brake pedal. It means if you hit the brakes in an emergency, the cruise control would NOT disengage.
To be honest, it wouldn't cost that much to buy a 4 pole switch.
Yep, on an auto the auto box ECU can also disengage cruise control.
And the auto ECU also looks for the pressing of the brake pedal via the standard brake lights contact (as used on all cars anyway).
The Auto ECU, when seeing the brake pedal pressed, signals to the cruise off via its own input.
So, its only really essential for manuals!
Beware though as it would appear that if you bugger with the brake switch and get it wrong then you can lock the brakes on (cant you Jimbob!) and that results in all sorts of screaching and grief