To me they detract and distract from the image itself an awful lot, and I find myself not focusing on the content of the picture as much as I would like to.
But each printer has the freedom to choose how they want to print and show their work. When you look at Diane Arbus's prints in museum shows, for example, the foundation has extremely clear instructions of exactly how the prints may be displayed in their frames, and the image borders HAVE TO be shown, and then a certain distance between the image edge and the over-mat border is mandatory.
To each their own. I try to rely on my pictures being satisfying enough, (to my own senses), that I don't have to rely on effects outside the image area to improve it. Of course somebody else will think that I'm a lunatic for doing so, or thinking that way, but that's how I feel.