I'm not sure if you're referring to a 'shared' SSL certificate or purchasing a certificate specifically for your site.

If you're planning on processing credit cards through your site, versus 2CO or Paypal, an SSL certificate that's specifically for your site is a must. A self-signed or a shared certificate may be almost as secure, however, it will always trigger the warning pop up in browsers.

You wouldn't want to make your entire site SSL, just a sub-domain where you process credit cards or other sensitive information. When you create a request, you would want to do so using secure.mydomain.tld or www.secure.mydomain.tld versus mydomain.tld. Since certificates are site specific, any time that you create a link to the secure portion, you must do so using the exact domain/sub-domain that you used to create the SSL request, otherwise visitors will get the warning pop-up.

If you're not processing credit cards or other sensitive information, an SSL certificate is unnecessary.