It does not make sense to use SSL on the whole site. No one cares about which page and who is browsing the website if there is no sensitive information.
The website will be rendered very slow if you implement SSL on the whole site. Web surfers are impatient, especially new visitors. If they don't see your pages load in a few (probably 5) seconds, they would just close the site, since there are so many results on the google/yahoo pages, you're thrown out of the sea.
However, I would recommend you secure the whole admin section of the website, which current zen-cart version does not do (it only secures the login page).
The way of doing it is easy, just go to the admin/includes/functions/html_output.php and modify the function zen_href_link
add a line to set the variable $connection = 'SSL';
but you might encounter minor problems on some pages since they were not originally written for the SSL environment. However, I don't see so many so far.