Well, in that case, it doesn't really matter if the water spills out as they use the nipples.The water under the fountain doesn't bother me as much as worrying that they get enough water. Since the fountain is attached to the fence there's no room under it for a tray or pan. They would step on it every time they got a drink. That water would get dirty real quick.
The dampness underneath proves they are drinking it, and by moving water through it more rapidly, they are keeping the water fresher.