Once you have https for admin users you need to have https on the front end as wellat least for the logged-in users as their authentication cookies will be sent in clear text when they access the front end and 3rd parties will be able to duplicate them and use them to get into the admin area for that user.
This do not require https everywhere but it is just the mentally easier option, and you don’t need to worry about some bug in some badly coded plugin or theme exposing your user’s cookies. For the option of SSL only for logged in users take a look at http://codex.wordpress.org/Administration_Over_SSL, couldn’t find anything more recent so you should use it as starting point for investigation.