A dedicated IP isn't a SEO factor which would improve your ranking, but it is a SEO risk to consider.
As a matter of fact, a shared IP could host thousands of porn website which would spoil the IP authority for instance.
Also, when you have a network of websites, it is recommanded to get a dedicated IP for each.
As for getting a seperate IP for each site, this is something that really annoys me - sorry -
Do you REALLY think there are enough IP's to go round if everyone took this view? even worse is the view
that each site needs a seperate CLASS C address.