Many web companies use a common, core set of javascript libraries and applications which use hierarchical CSS architectures.

If the live server is used for development (which is a very cost-effective approach), thousands of requests are made for the same files.

To reduce this, common components can be sourced from a single point, which the HTTP headers set to cache them with far-future expire dates.

In addition to reducing bandwidth consumption during the development phase, it also reduces it for live sites.

If there are related sites on the same server - the benefit is extended.

This is a small CDN (content delivery network), which allows distribution of customized files for a company, or custom code. It also simplifies management of the common code core, since updates only have to be made to one file.

This saves disk space as well, and speeds page loads.

If there is an issue with having content from a different URL, rewrite rules can be used to hide the true source of the files, while retaining the advantages of a single source.

With additional creative configuration, an application can be used to support multiple accounts. This is riskier, and I don’t recommend it - unless you can be sure you will never change the application. I would try it with this blog software (b2evolution), because from what I’ve seen - it would be very successful.