Pros : I should have thought of this. This makes keeping your piece square and the right size easy. By using this system, your piece will always fit your frame.
Cons : you can not have closed corners for any other shape than rectangular. I cut the corners of the side that gets pinned to the board at 45 degrees. This was to accommodate the piece I was making that is octangular.
Other Thoughts : The instructions say not to use with a wood board. The pins pull apart when you try to remove them. I used a piece of "green board" which is a type of drywall board found at any big box hardware store. It held the pins well and they were easy to remove. It also took the heat of soldering with no problem.