I've been designing and developing web sites since Netscape was in beta and I had to use Trumpet winsock to access the internet over my 28.8k modem. Since the very early days I've always used underscores in lieu of spaces for my file names and reserve the hyphen to replace punctuation such as colons or well, hyphens (I remove the space around dashes instead of using underscores as it looks weird this way).
While developing a site recently in a new (for me anyway) CMS, I found they had no way to choose underscore or dash instead defaulting to dashes for generated URL slugs. Now, I could manually retype the slug every time but this seemed tedious and error prone, and I hate tedious and error prone. I went to their forums and asked if it was possible to change this setting. The reply was a "No, why would you want to, Google says to use dashes", and they added a link to a Google page.
Now, many are already probably screaming at their screens now, "Idiot, SEO says to do it this way!!!" to which I say, why? I did a little digging around and all the articles I turned up that offered any "proof"of this being the way to do things all pointed to this page. A single page on the internet that only states a preference, lacking any reasoning behind it. No offerings of how or why it will improve SEO. No mention of why Google prefers things this way or if it will hurt your site ranking if you use underscores. Nothing, Nada. I did find an article or two that suggested this reasoning is out dated and no longer relevant, which leads me to my big question. Does it matter and is there any evidence other than this one page that states why dashes are better than underscore other than personal preference?