Wait, what do I mean "RIA seems to defy definition"? Well, the definition of the term RIA seems to be shifting continuously. It certainly no longer just literally means rich internet application. I have said before that RIA was becoming a synonym for website, because nowadays, people expect you to build an RIA when they ask you to build a website for them. Just yesterday, I attended a seminar about RIAs, and the keynote speaker was of the opinion that an important aspect of RIA is "cross-platformness". And I can only agree, because the capability to build an application only once and being able to deploy it on many operating systems and devices is definetely a big development and maintenance cost reducer: win-win.
Interestingly, the definition of the term Web Service is undergoing a similar process as its meaning is also becoming broader. Well, that is not entirely true, because the definition of Web Service is depending on the perspective of its beholder. Seen from a business perspective, a web service is a course granular thing that reflects a certain business process or transaction, for instance "payment". Seen from the perspective of a software architect, a webservice is a programming interface that provides a set of operations that can be invoked from anywhere on the web. Seen from a consumer's perspective, a web service has a much broader meaning: a service of a company that is rendered to them through the web, for instance "online banking". Each of these notions is essentially true.
Did you notice how close the perceived meaning of "web service" comes to the meaning of RIA when seen from a consumer's perspective? In any case, an RIA can significantly improve the quality of the service that is rendered to the customer, because the rich capabilities of RIA technology allows you to develop appealing and responsive web applications. Add the above mentioned "cross-platformness" to the equation and you get a no-brainer business case: RIA technology improves customer satisfaction and reduces cost. And if you pair RIA development with an Agile methodology (also a no-brainer) and rapid design & visualization, you can have your customers experience the first version of your cost reducer and customer satisfier within the month. No kidding.