In a "pure" MVC environment, there wouldn't be much point. You put a property value in,
you should get the same property out. End of story.
In the Real World, however, models aren't always pure. A very common digression at the moment is the
JSF Backing Bean. JSF Backing Beans are
mostly models, but they also typically have logic in them as well - a characteristic that causes a lot of people to incorrectly label them as Controllers and not Models. A Controller, however, is what synchronizes the View and the Model, and the most common type of logic you'll find in them are JSF Action methods, which are not UI controllers, but rather the interface logic between the JSF Model and the rest of the application.
These Action methods very definitely are testable, since the JSF design was based on that premise. Well-written Action methods have little to no JSF-specific code in them, and get most of their operating data from values previously injected into the model. In turn, they may update the model data, which can then be queried by the test.
I do this fairly extensively, in fact.
Other things that can require testing in a model are side-effects (where setting one property cascades into changes to other properties) and converting set/get methods where, for example, values get scaled or modified (such as Fahrenheit/Celsius). Experience has taught me that the fewer instances of these, the happier life is, however, test or no test.
Finally, of course, some objects have a (re)initialize method that sets properties to default values. It can be useful to test and make sure that they do.