Some clients often ask me if they should obtain a few domains and link them together as an SEO scheme to get ranked on the first page of search results. My answer has always been: No. And Matt Cutts has covered this subject nicely in his video:

YouTube video

The scheme is not just spammy and may get you off of search results pages all together You could be penalized further by violating duplicate content rule. If you have several domains pointing to the same website, you are essentially duplicating the same content or at least it appears like that’s what’s going on. The violations together can wipe out all your SEO effort and thus your investment for your online presence.

Although it is not likely that anyone will have 20 domains on similar topics but one could have several domains using different city names all pointing to the same website to imply they do business in those cities and to get ranked for localized searches. This will also fall into the same scenario as above and should be a strong no, no.
Jim Gharib