i'd imagine it's important at first when your website doesn't carry any authority or SE weight, in the instance that it's new and pages need to be indexed for the first time and a sitemap would definitely help with that. but once you get your PR up and organic traffic flowing about, google bots would have no problem crawling your pages. that is if you have a proper link structure. which isn't too difficult to setup.
to summarize, sitemaps do help, but you don't necessarily need them. you can provide both XML and HTML sitemaps, neither would hurt. Matt Cutts stopped using an XML sitemap and only has an HTML sitemap.