The first is scalability and the other is flexibility. Beware the ASP company that says it can handle any volume of users at any time. That's ill informed marketing talking. Monitoring and managing how well a service performs when it goes from hundreds to thousands to tens of thousands of users per day is one of the most critical aspects of delivering reliable service. ASPs fall into categories ranging from service to individual users to those that cater to the demanding needs of global enterprises.
Crossover is unusual.
Also, realize that when you use an ASP, you're getting the benefit of a very cost-effective service. However, if you have very specialized needs and can justify the investment, it's likely that your application would be better off developed as a custom solution. ASP services aren't the cure all for any type of business need, but they certainly lower the bar for adding more interactive, value-added services to your intranet, extranet, or public Internet web presence. It comes down to knowing what will support your business objectives and now knowing more about what is available to help you meet those goals. Bill Ringle works with organizations that want to use the Internet to grow their business.