This one looses me. Who is it for? Are there really people out there using UML who need something they can put in their pocket to use when they need a memory jogger?

The concept of Pocket guides is fine but it does not make sense to produce them for all the range of subjects that O'Reilly publish. There just aren't that many facts that you need to remember when using UML. If you are familiar with UML you do not need this book and if you are not then you need something much more detailed plus some study time. There is nothing more to say.