I agree that a degree is not necessary to be a good consort. There are plenty of people out there who don't have a tertiary eduction but who have been very successful in various fields, and having a degree is no guarantee that the holder will spark or sparkle in any way.
I do, however, think that art history covers lots of topics that will add an extra dimension to Kate's understanding and enjoyment of her role. As well as painting, sculpture and architecture, she will have acquired a knowledge of world history, historical personalities, philosophy, religion, sociology and all the other subjects touched on in such a course. Instead of just being aware she is in a beautiful old mansion full of antiques, she will be aware of the style of the building, the political circumstances at the time it was built, and will recognize paintings, etc. she sees there. Assuming the owner or occupier has a similar interest in his/her surroundings, she will be able to discuss these things with them, or at least afterwards with William. When she travels, she will have advance knowledge of some of the history of the places she visits. I think that her education will be a big plus.