DukeofRealms, there are several good reasons to run HTTPS on all content, not just authentication components both for the users of the site, as well as the owner of the site.
If your site is only running HTTP for all content, you open up your users to MITM attacks (manipulating the webpage content to add malicious scripts, etc), particularly if they connect to rogue WiFi hotspots when out and about. As a simple example, I could manipulate the sign in button to send username/passwords to my server via JavaScript as well as the regular sign in scripts.
Something worth watching is the Google I/O presentation on "
HTTPS Everywhere" [45min Youtube].
If the security side doesn't interest you enough, perhaps the fact that it is used as a
ranking signal in Google Search, increasing the probability that you'll be ranked higher in Google searches will help.
Dislaimer: While I work for Google, I don't work in the Search team and don't know anything that isn't public about how search ranking is computed.