One of the advantages is just as you say; you don't have to be punished for something someone else is doing on that IP..
You can check if an IP is listed in spam databases using this tool:
http://www.dnsstuff.com/tools/ip4r.ch?ip=
You can also find links there to how you can get you IP delisted if it's already marked as spam.
Other advantages are that you can
have your own SSL certificate and be able to use anonymous FTP. And if you have several sites, and manage to get different class C IPs then you may have some advantages if you interlink those sites since SEs checks IPs and may disregard/don't pay much attention to links coming from the same Class C IPs. Other than that I'm not aware of many advantages of having a dedicated IP...