On very wet/contaminated runways it's more about technique and preference whether to use autobrakes. Manual braking provides better "hands-on" directional control. Because the autobrakes don't "know" whether the airplane is on centerline, or off centerline, and by how much; nor does the autobrake logic correct for drift. Check out the following incident report about a 767-300 departing a very wet pavement.
Sliding Off The Pavement 