Now that WHM v11.38 no longer requires a dedicated IP address for a SSL certificate and cpanel doesn't support IPv6 yet, why would one need a dedicated IP for a web site?
cPanel servers by default send all accounts/domains mail through the "main shared IP." If you have an extra IP you can configure exim to send one or several domains mail through the alternate IP, that way if your main IP becomes blacklisted for spam, the domains mailing from the secondary IP are less likely to be affected.
Yes, cPanel 11.38 allows for the installation of SSL certificates on a shared IP address provided your server supports SNI (e.g. CentOS 6). The 11.38 release notes are available at: