Interesting concept. The issue that I see is that you would have to be exact in putting the holes in a picture frame, and then just as exact in putting the hooks on the wall. The adhesive backed mounts would help, but if you shuffle the art on your walls frequently, this could be a nightmare.
My initial impression is that this is a good idea. Like Tim, I was thinking that you'd have to find the center of the piece that you wanted to hang (not a big deal), but also potentially find the center of gravity as I've found with hanging certain "things" around the house. But after thinking about this some more, if the "Peg" fits snugly in the pocket hole, it may not be that big of a deal if it's slightly off center. If you use 2 of these, one on either side of the frame, then that should obviously help as well.
I may have to try a few to see how well they work. It may also allow the piece to fit more snuggly against the wall.
I kinda think $5.00 for 4/ea is a bit steep though.