An unfamiliar favicon lets me know I am not somewhere I normally would be, and to regard the site I am at more suspiciously. Without familiar favicons, I would treat every site that way before looking at the page more thoroughly. Obviously I do not rely solely on favicons, but they are a good initial way of orienting myself when looking at tabs.
In other words, the favicon is only useful for familiar sites or for knowing a page isn't from a familiar site. It's of absolutely no use in any other case, whereas a site name or a domain in the title tag will actually tell me what the site is in all cases.
