A single website is more connected and credible. It is more consistent and cost effective. It is easier to manage and measure.The Web is a network and the most important law of networking is to be connected. A large website is not necessarily a strength, and may in fact be a weakness, because people waste time there.
The number of links (connections) you have is nearly always a positive thing. This applies to both internal links and links to your website from third-party websites. (The only type of links you should avoid are links out of your website. Only link out where you have a compelling reason.)
Getting linked from third-party websites is crucial to success. The more links you are getting the more credible you become. (Links from quality websites enhance credibility far more than links from low quality websites.)
If you have multiple websites then each website has to build up its own set of links. Links to website A are not shared by websites B and C. Links to website C are not shared by websites A and B.
Multiple websites weaken the potential power of the overall organization on the Web. Each new website dissipates that power, as the Web sees a range of websites, each with a relatively small set of links. If the organization behaved as a single website (as, for example, Microsoft and Apple generally do), then all incoming links would converge under a single website address, thus making the organization more powerful, visible and credible.
Someone once said that "you get the intranet you deserve." Certainly, the intranet says a lot about the organization. Many organizations have multiple intranets with little or no consistency of design or links between these disparate websites. This says that the organization is not a very cohesive entity, that it is in fact a loose collection of disparate entities.
Organizations that are more cohesive and coherent tend to be more effective in the long term. Organizations that encourage collaboration tend to be better able to respond and adapt to constant change. Collaboration depends much more on culture than on technology. If the organization has multiple websites with multiple different designs, it may reflect a culture that doesn't want to collaborate. It may also, however, reflect poor management. I know of one organization that went from 600 intranets to a single intranet, and received a 90 percent staff satisfaction rating as a result.
A single website architecture is more cost effective and easier to manage. A lot of money is spent on creating new designs and re-doing old ones. This money could be better invested in creating quality content, and in ensuring that the website is well maintained. A website with a single architecture is easier to measure, and this is an important point. Many websites have primitive and unreliable metrics, and this makes management all the more difficult.
There are times when you will need to create distinctively different websites. If your organization does radically different things, then there is less benefit in having a consistent design and architecture. However, most organizations would benefit from a single website approach.
Gerry McGovern, a content management author and consultant
, has spoken, written and consulted extensively on writing for the web and web content management issues since 1994.