Well, you could -- but the salt level may vary in different brands of butter, so you might not get the same result all the time. Plus, even if you always use the same brand of salted butter, it could take a lot of experimentation until you get a salt level you like.
From the professional recipe writer/editor's viewpoint: the recipe was tested with unsalted butter. The person who wrote it meant it to be made that way, and knows that the outcome should be as promised. Once you start changing things, don't expect the recipe to be what the author intended.
PS: if you normally have only salted butter in your kitchen, try using unsalted for a change: it tastes a lot fresher, and allows you to adjust the salt level in all your food much more easily.
"Notorious stickler" -- The New York Times, January 4, 2004