They can be an impediment to graduate students trying to move beyond their field and move nimbly between fields. Even a team-taught interdisciplinary showcase is going to leave out many students for much of the time if it plays out serially over a quarter or a semester. Think instead about short, focussed, one or two lecture-sized segments with supporting materials. The idea for developing courselets came from two sources. One, truth be told, has nothing to do with interdisciplinary teaching per se, except for recognzing the problem of prerequisites and varying student backgrounds.
Time is short.
Can't do it all. There are students from all over who need to know about Fourier transforms, each schooled in their respective disciplines, and they all show up in my class. The class is very mathematical, but the mathematical background of an entering EE student is not the same as one in Mechanical Engineering, and so on. For many students it's not even material that's missing as much as it's material that's been misplaced, or seems somehow oddly mislabeled. Have a courselet on what they'll need and how they'll need it.