If you read through googles gobblygook, you will see the mention that you can use your company site map also and it will not hurt you to have a html one and a xml one. I know I was indexed mega fast using theirs. Say, 5 days after putting up our site we have every page indexed. The results are less than luster but hey..I think it's worth it.
Site maps are usable for both SEO and for a user that doesn't want to fumble through all the menus of a complicated site to fidn what they need. Google knows this and gives you a nice bonus for this from what I understand.