Designers are often in a situation where they are moonlighting and don’t have time to set up these types of scenarios - the client wants a site redesign YESTERDAY and don’t care to listen to our use cases. Let’s face it…a well designed site takes longer to create and could potentially cost the client more money in the short-term. How do we handle client expectations while delivering a quality product from a usability perspective without staying up until 2am every night and under-charging for our services?
I think the use of UML really depends on what type of project you are working on. If it is a clean cut website with little database connectivity/complex features then I believe it is fine to skip the UML stuff if time is a factor.
But if they want a complex site with more features and a large amount of database interaction, you must make it clear to the client, the importance of the UML diagrams in a well-planned system. Explain how it will take more time, and more of their money if the system has to be reworked during the development phase.
For more information on this subject, I suggest Rapid Development by Steve McConnell. It is an easy read and I found it very interesting/helpful.