The original 50' screen height, later amended to 35' for heavy transport aircraft are ARBITRARY, but reasonable values. The 15' screen height is one of two major concessions allowed by the regulatory authorities for wet runway operations, the other major concession being the allowance of Reverse Thrust as a credit in the Accelerate-Stop calculation.
Basically, the two concessions are economic in origin, if aircraft had to conform to the normal requirements for wet runway operations, the payload penalties would be considerable. I had first hand experience with this in the certification of one aircraft, where the regulatory authority accepted the 'Reverse Thrust concession', but insisted upon the 35' screen height being retained. It resulted in about 25% reduction in payload on most runways.
Under some unusual circumstances, use of the two concessions will result in a Wet Runway RTOW exceeding that for the Dry Runway. It was necessary for me to include in the Ops Manual, that, for Wet Runways, the lesser of the Dry and WET Runway RTOW be the limiting weight.
What The Flying Cokeman says about wet runway performance is absolutely correct, but as a consequence of conforming to the wet runway procedures and legislation, not as a direct consequence of wet runway operations per se.
I think that kenparry is close to the mark in asking "Does it reduce safety?". Yes, most decidedly it does, it removes my two quite small reserves of safety in both the Accelerate-Stop and the Accelerate-Go cases. In the Accelerate-Stop case, my small stopping reserve, reverse thrust, is consumed in it's entirety leaving nothing in reserve. In the Accelerate-Go case, the screen height is reduced to a mere 15', almost nothing at all.
Even with the elimination of the reasonable margins of safety when using wet runway performance, I still see F/Os reaching for the Wet Runway RTOWs when the runway is, at most, a little damp. There seems to be a horrendously incorrect belief that these data are more conservative. In GENUINE wet runway conditions, of course, you'd be a damned fool to use anything but the Wet Runway data, because other factors such as reduced acceleration and reduced stopping capability are considered.
I hate those days when the ATIS says "Runway Wet", and there's just a few little puddles on the runway or it's a bit damp. Then I'm obliged to use the Wet Runway data, because at the investigation............. (You fill in the spaces between the dots)
Regards,
Old Smokey