Yeah, pretty much what all you guys have said, covers what I'm going to say. Haha.
When my Mom got divorced she kept by Dad's name for the sake of me. She didn't wish to have a different surname from me and felt that I shouldn't have mine changed until I was old enough to make the decision myself. (Which I did at age 18. Hehe. Course then marriage changed it again...*sigh*)
And I think Palabravampiress got down the other side of it; business recognition. If Joyce had stated her career in art (history or otherwise) with her married name then it would make sense for her to keep it as it is that name that retains her business standing. I know a few people who, when they've gotten married and chosen to take their husband's name, it's caused quite an issue in the working world trying to reaffirm with every contact that yes, they are the same person etc. Complicated stuff, hehe.
Or who knows maybe Hank actually took Joyce's name? And her second name actually is Summers. Not likely considering what impression we got of Hank's character, but hey, it's possible.