I never use quotes because you have to Shift and press the single-double quote key. So that's four extra keypresses for each URL. And I have always hand coded every piece of html. In the old days when I was getting 100s of thousands of hits a day on a page it would save on bandwidth and loading time. Nowadays I do it because it works.
Same way I never use /body and /html tags - also I never close any formatting tags inside frames because a /td closes them all for you. I do loads of non-standard stuff but it works in all the browsers - and Google still crawls them fine too
Oh, BTW - they have now amended xml-sitemaps.com after I emailed them - and it works great with or without quotes now.