its only occasionally in very sloppy conditions where narrow tyres are beneficial. and even then there has to be a firmer surface lower down.
Most of the time i need more grip when clambering up steep climbs, narrow tyres seem to spend all there time digging deep ruts in the hills but not getting me very far up. But you can often pick you way through rocky surfaces with narrow tyres, with big tyres you just seem to end up clambering over stuff not paying too much attention to where rocks are.
