FWIW, I'm registered in an interdisciplinary department, and I think SGM is right that the hierarchy doesn't like it - I don't think it's any accident that this department is also far, far more feminist and female-friendly than the very, very strictly-defined single-discipline department I've just left.
I think - repeating myself, sorry - it's back to binaries again. Patriarchy likes binaries; they're good tools of exclusion. So when people come along and say they'd like to think across the boundaries, that is very threatening.
Just an anecdote, but I find it fascinating how often the same boundaries that are being enforced today are also being used to dismiss the studying of women. So, for example, in my area a lot of books written for women and taught by women are seen as being partly in the discipline of Art History, partly in Religious Studies and partly in English Lit. - so when I tried to study them, I was restricted because I was told there was no-one would could supervise me adequately since I worked in a strict English Lit. department. This was put forwards as a fact, with no suggestion also has a pretty huge and damaging effect on the study of these books, which are so important to understanding women in history. People were saying it was just an arbitrary boundary issue, because I needed help from people studying other disciplines - but the result was to push women's books into an area where they're harder to study.