Staying dry and therefore warm, as it was mentioned before, has the benfit of keeping you sharp and in focus, so you can fully concentrate on riding, instead of on, how wet and cold you are. The biggest factor in wet conditions is the grip or the lack of.
I join the posters that claimed the grip being nearly the same as in dry conditions, except on occasions like, oil in the intersections, painted white lines, muddy run off on country roads. Once you find out that the bike won't slide as soon as you lean it, you can relax your death grip on the controls, being really tense, makes riding a deadly affair. The only thing shitting me about riding in the rain is the visibility, i wear glasses and if they don't fog up, the bloody visor will!