Google has a detailed blog post named Open redirect URLs: Is your site being abused? The post goes through the issues with having “open” redirects on your site. Open redirects can lead to spammers abusing your site and ultimately hurting your rankings in Google or other search engines.
The post goes through the various methods spammers can abuse your redirects, ways you can check if your site is vulnerable and ways to patch your redirects so they cannot be abused by spammers. This is a must read topic for webmasters who run any heavy dynamic site.