SEO stands on three basis:
- Content (what you write/publish)
- Relations (how others give you importance)
- Structure (how you make your website readable/crawlable)
All three are important. Really important.
Sitemaps are a best practice, in order to keep your content indexed in one unique place. But please, make this in a smart way.
Make only ONE xml sitemap and derive the HTML sitemap from it.
Closing this question, please note that any website where you do log in to interact isn't really intended to be indexed by crawlers. They are made to be used, not just read (do you think google, yahoo, bing, ask... worry about SEO? Facebook, Friendster, Twitter, Youtube, Orkut...).