The quick answer is "no, having multiple domains doesn't help your search engine placement".
The longer answer: pointing multiple domains to the same website doesn't impart any search engine benefit, because Google is smart enough to know that they're all pointing to the same place. Google picks the oldest URL and designates it the "primary" domain. All other domains are disregarded. If you had dreams of buying dozens of keyword rich domains and suddenly ranking #1, you'll need to rethink your plan.
But what if you already bought 30 domains? Now what?
If you're already sitting on a treasure chest of keyword rich domain names, I recommend setting up lots of mini websites on wordpress and linking them to your primary site. Each "slaved" site should contain a few pages of original content and a handful of embedded links to your main site (or subpages). These miniature sites will impart a small amount of search engine credit to your main site, especially if you continue to post articles and updates on them. Eventually, with enough work, you might end up with multiple sites on the first page of Google!
This strategy is time consuming and I only recommend using it if you're in an extremely competitive area.
