One of the reasons using a dedicated IP can have a positive influence on rankings is because the engines take into consideration how fast your site loads in comparison to other sites. If you’re sharing your IP with 500 other Web sites, the server, like all good customer services departments, will deliver the files in the order they received them. If there are 10 people ahead of your visitors in line, they’re going to have to wait, resulting in a slower page load times and frustrated users.
Sharing an IP address also doesn’t allow you to control who your neighbors are. If you’re sitting on the same IP as a gambling site, a porn ring, a Viagra dealer, and a priest, and one of those addresses gets banned by the search engines, you’re banned too. Search engine’s don’t just ban domains, they ban whole IP ranges.
