Actually, at the college my DS will be attending, it's a way to combine various interests into a personalized major. For example, my ds wants to major in music, but he wants to combine some theater and contemporary vocal music with instrumental performance, sound recording and music business. Sounds like a mess, I know, but within a framework he'll set up with his advisor, he can take classes across a couple of programs and be set up to go into working possibly as a talent rep and performer.
If you wanted to combine, say, art with social work, that might be difficult to do in a traditional program, but they do have a real value together. With an interdisciplinary degree, you would have less trouble taking classes in the fine arts school and the liberal arts for the social work, and not have to take some of the classes that would have less value. It works out to be more than a minor, but allow you a real interworking of interests toward a specialized degree. I think many kids today have a much more specific idea of what they want to do than some of us oldies did when we went to college, thanks to better career counseling and exposure to more careers through internet research. The traditional degree has a place, but that place may be getting smaller and smaller over the coming years.