I believe that makes for dedicated affiliates, and as a by-product it reduces the number of sales lost to the cheapskates who just want a discount on the product. So far it has worked well.
But many other marketers actively promote their affiliate access without any strings attached, and it's easy to get signed up everywhere.
So am I being too tough? Should I open up my affiliate world to everyone and go for a mass market approach, or stay selective as I am doing?