I've used the Fobus paddles for a Sig P229, S&W 3953/6946/5943, Sigmas and a Glock 19.
I traded for the Glock to a friend of mine, and I carried/shot it for about a year before trading it back to him. I used only a Fobus holster, but did a LOT of training with it. By the time I traded it back to him, he though it was a different gun. It was practically new when I got it, but the Fobus had worn a fair amount of the finish off the slide.
Yes, the Fobus will wear the finish, but it takes many, many draws to do it. In fact, I went through a few Fobus paddles for the Glock because they got worn out from all the training I did with them. They didn't have much retention left to them.
The finish wear is nothing I've ever concerned myself with. I buy my guns to use. If they get holster worn, it just adds character.