I read a good case study before... hosting a quality site where the ip is shared by many low quality sites, and these sites are getting auto-penalties and manual reviews anyways, and then their quality site got knocked down, while a similar site they set up on a dedi ip address went fine... so to G they're just saying "well this all looks like a shitty bunch of sites on this ip address, might be easier to penalize all of them, even if some that don't deserve it get hit, it's worth it"
On shared hosting, a lot offer you your own ded IP address for $2 more per month. I think doing that is worth it. You might be on a server with these sites, but a separate IP puts some distance between you and shitty sites.