4imprint.com - This is a leader in promotional products industry, there are thousands of keywords in this industry/niche and they rank on top (top 10) of organic results for every keyword. They don't have a sitemap at all, no XML no HTML sitemap. How do they rank higher? When we say sitemap is the most important for indexing/seo etc etc.
Yes, sitemap is important for indexing. For SEO, not so much. It is all about link juice. If your website was junk but had a backlink from CNN.com or other trusted source it would probably rank high with very poor content, no sitemap, etc.
Sitemap isn't a requirement. The whole reason behind a sitemap is to give Google one page they can visit in order to obtain links to all the other pages around your website. If you have things linked in a way Google can still find all of the pages by crawling through your content, then a sitemap really won't matter. It's all about having your pages linked to from somewhere so that Google can find them!