Martin Fowler says the following:
Whether a property is implemented as a field or as calculated value, it represents something an object can always provide. You shouldn�t use a property to model a transient relationship, such as an object that is passed as a parameter during a method call and used only within the confiness of that interaction.
I can�t really understand what he wants to say in the second sentence: what is the problem with transient relationships?
If you put transient relationships in the model, reader of model is overwhelmed with information.
Also transient stuff is inherently dynamic. An instance member is allways "there" as long as the class does exist, though it might be null. A transient variable has a much shorter life-cycle.
Transient stuff gets focus in types of uml models which show dynamic behaviour like interaction diagrams.