I work on corporate level custom webapps. The main two areas where you have to be careful to design things correctly and to be able to convey your ideas to end users (in clear, unambiguous terms) are - in my experience, at least:

- Database design

- (Web) Interface design

Oddly enough, UML provides zero support for either. So it's always DBA stuff on one side and mockups/wireframing/prototype on the other. Especially to discuss user-front things (i.e. how the application will look and how you will work with it) UML is completely useless.

These are our primary concerns, and UML does not help at all. Use cases are too abstract, and the rest is of little value for the users.

So despite my attempts at learning it, I was never able to get any real value out of it.