Purfling works by interrupting the growth of cracks since they like to travel along the grain of the wood. If that grain is broken up in some way, say by inserting another material into it, then the cracks a going to have a much harder time spreading.

Cracks are kind of like knocking over a line of dominoes. You tap one and the rest go as well. If you remove a couple of the dominoes and put a heavy book in the line instead then only those dominoes up to the book will fall. The rest will be safe.

It is a bit counterintuitive and under certain conditions the purfling could be a problem. Under the conditions a violin is normally exposed to, however, the bumps along the edge that create little cracks are far more of a problem. Those are what purfling is designed to protect against. I could elaborate more later but now I'm running late to class.

My own fiddle is purfling-less and has some cracking across the top and back that followed the grain from the edge. I'll post pictures sometime.

@kevin M.

I was probably typing while you were posting. I like that paper analogy.

In the post above I mentioned there would be cases where purfling might not work so well. Such conditions would include those where the wood was put under extreme tension or torque. This is why a traditional hunting bow would not have any sort of grooves cut into it and be retired on showing cracking. Under repeated loadings of 100 lbs, or more, the bow would eventually blow apart.

Violins, however, are not subjected to such extreme stresses and are, comparatively speaking, fairly static. Thus, you're not trying to get the absolute strongest wood out there but rather a wood that will sound good and support normal playing. Then the goal is to protect that wood from normal wear, which brings us to Kevin M.'s post.

In short, yes, purfling weakens the wood somewhat. However, this is a design choice that is acceptable because it a.) is still within the necessary tolerance for the violin to function (you don't need the absolute strongest material/design, you just need the material/design that is strong enough for the job) and b.) actually serves to protect the instrument by limiting the spread of cracks due to normal use.