Some of it may be due to laziness on the school's part to combine the classes, but there are also benefits to the dental student: you get a deeper medical education, and you get classes that otherwise would have never been offered to dental students. Back as a D1/D2, we had Dr.Oz come in and talk to us for cardio (he is an author and was on Oprah), Dr. Gershon (author, appeared on Colbert Report) for the gut, and Dr. Kandel (Nobel winner) for neuroscience. Dental schools, in general, tend to be less well-organized than medical school, so there is a BENEFIT to piggy-backing on the med classes. Most of our dental classes, though also well taught, are not nearly as organized and well taught as the combined medical classes were.