Personally, I will never pat my horse, I will stroke them, or pet them. I want my horse to learn that any pressure whether it be constant pressure or inconstant pressure means something and not "you did good". I have yelled at many many people that have came up to talk to me, then started patting my horse. My mare could care less, I'm sure you could stab her with a pen and she'd give you the "I love you" look but my gelding (and my other horses), you pat him and he is moving away, trying to figure out what that means.
Just was curious to see what everyone else thinks, I don't think it's right to treat a horse like a dog. We pat a dog, a dog understands that, and yes a horse can learn to understand it, but does not mean they enjoy it.