You gain stability, you may lose traction if only one wheel is powered.
http://www.youtube.com/watch?v=jA_I6jH7DY8
http://www.youtube.com/watch?v=EgbIG1FUB2o
This is riding my Worksman front-loading tricycle on hard ice and thin snow, basically slick but not boggy.
__________________
"be careful this rando stuff is addictive and dan's the 'pusher'."