I doubt if priorities have much effect. I found that it didn't seem to matter what they were set to. But I also found that it didn't matter what was listed in the sitemap either, because Google spiders and indexes what it wants. Now I use a sitemap that is easier for me to read and update... one without priorities and without indents for every tag (one line per item) and that seems to work fine.
I'd just go with the documentation [sitemaps.org...] and use the options as they're defined and intended since I can't remember any search engine ever saying xml sitemaps were used for ranking purposes rather than helping with discovery especially on sites that are difficult to crawl and possibly "which URL to show" when there's a question, even though (incorrectly) some "SEOs" may have.
Oh, just answered the question and saw some of the previous posts earlier, but didn't see JimBeetle's until now, so "what JimBeetle said".
I will add: It's really sad to me to see search engines get the blame for what some "SEOs" (used loosely) incorrectly state. Search engines (their reps / documentation) haven't ever to my knowledge stated xml sitemaps could or would be used for ranking purposes, the only place I've heard that is from "SEOs" who don't know wtf they're talking about.