Multiple Domain Names
Having multiple domain names, why and why not?
Some people are under the impression that having more than one domain pointing to your web site is extremely beneficial for search rankings. In all honesty there is only a limited benefit to even owning those extra domain names. First of all, you should have one primary domain name that you use, market, spell out to people over the phone etc. This domain is your be all and end all (unless you have a huge corporation or have distinct differing products/lines.
When you should have more than one domain name:
Distinct brand or products: Take for example www.coca-cola.com and www.dietcoke.com. Pretty self explanitory.
When you can have more than one domain, but for the same web site:
Rebranding or restructuring a corporate image. Example, Northwest Crime and Social Research (www.nwcsr.com) rebrands itself Looking Glass Analytics (www.lgan.com). Ideally this should be a flip of the switch where Domain A has a permenant (301) redirect to Domain B. There might be some uptime with both before the change over to prepare current site users.
When you should buy domains but maybe not use them:
Doctor A has a practice in Podville, KD and uses www.podvilledentist.com as his primary domain. Doctor A may also want to buy www.kddentist.com or www.podvillekddentist.com or www.dentist-podville-kd-doctor-a.com to keep his competitor Doctor B from having a domain name too close to Doctor A's. If necessary you can use a 301 redirect to point these domains directly to the primary www.podvilledentist.com.
Do Not however have these as separate domains, which look like exact duplicate copies of full web sites. Search engines see this as spammage. This is really what you want to stay away from.
What to focus on:
Your primary domain. This is the core of your web presence. Use this to market distribute and brand yourself out in the world. Send out press releases, artices, news, advertisements, and anything else you can think of that has your web site linked on it. You will get better rankings, familiarity, and traffic with one core location for your web site than spreading yourself wide and thin. Focus is a good word to remember.