I recently did some work for Suzuki on their SuzukiCycles.com domain with another agency. One of the things that I noticed was that Suzuki splits its traffic across 4 domains:
As a result, the domain with the most traffic is SuzukiCycles.com with about 87,000 unique visitors per month. To compare, Kawasaki, which does not split its traffic across multiple domains, gets about 237,000 unique visitors per month.
As you may know, the amount of traffic your domain gets is very important in helping to determine your search rankings. So, I decided to take a look at how Google was placing these 2 companies in their search engine results page for the keyword “motorcycle”:
As you can see, Kawasaki ranks 3rd, just behind Harley Davidson and motorcycle.com, and Suzuki ranks 10th. Now there are many factors other than traffic that contribute to page rank including inbound links to your site and the overall traffic of the sites that are linking to you. However, if Suzuki were to combine all of its sites under one domain it would actually have more traffic than Kawasaki (~243,000 unique visitors vs ~237,000). This would certainly help their page ranking in the many search engines around the web.
The bottom line is that companies should use subdomains (for example, motorcycles.suzuki.com might be a good one in this case) if they wish to give a certain line of business a unique URL rather than splitting traffic across entirely different domains. Splitting traffic degrades one of the most basic and easy ways to get traffic to your site via organic search. And, best of all, it costs nothing to use one domain and probably decreases the complexity and cost of maintaining multiple sites/domains.