austtr - 8:06 am on Jan 7, 2013 (gmt 0)
I going to show my age here.... but if I remember correctly, the underscore dates back to the old DOS operating system which did not accept hyphens or spaces as valid characters.... hence the use of an underscore to connect separate words in a file name.
When search engines started collecting data into their databases, its logical to assume that there would have been old DOS era documents somewhere in the mix so underscores would have made their way into the internet era. Devaluing a document/page simply because it has an old style file name seems a bit over the top.
I'd put this in the myth category.