What are the benefits of having an unique IP address?
The big one is that you need a unique IP address for some SSL/TLS implementations. As pointed out in the comments, no version of IE on XP can do this, which is the biggest offender.
Also, if you have an application that needs to reference an IP instead of a DNS name, you'd need it since your shared host is likely configured to ignore requests to an IP.
Blacklists (SEO, mailings etc.) are often based on IP, so having unique IP will reduce propability that you will suffer for someone's other actions.