SSL is a must for any eCommerce website, but with site security being an increasingly important ranking factor it's a good idea to secure your entire site using SSL whether you are taking transactions or not.
I'm pretty horrified when I see Wordpress and Magento log-in pages on public websites that are being run without SSL. It doesn't matter if your browser turns your password into little dots in the text box if it is being passed across the internet unencrypted.
There's no good reason not to have SSL in fact, especially now that SSL certificates are available from Let's Encrypt. Your website should be running SSL, no exceptions and no excuses.