Now, if the argument had been that the student did not possess the skills to deal with the two areas under discussion, there might have been room to talk. Even then, why not require the student to take 'x' courses or, in some other manner, gain the necessary expertise to undertake the project?
There is something fundamentally wrong with this type of academic 'territorialism.' Why, for example, was a prominent scholar of Halakhah once pilloried for having the temerity of addressing the interface between Halakhah and mysticism, on grounds that were reminiscent of R. Eleazar b. Azariah's retort to R. Aqiva (B. Sanhedrin 38b): 'Akiva, what have you to do with Aggadah? Confine yourself to Nega'im and Ohalot!' If one acquires the necessary skills, why not?
Specialization is a fact of contemporary academic life. However, there is a difference between specialization and compartmentalization. When I was growing up, a well known piece of advice was: 'Be a Jack of All Trades, and a master of one.' I believe that this should be the guiding principle of scholarship. This was one of the most important lessons that I learned from the late Prof. Isadore Twersky ז"ל. Knowledge is unified, and thus inter-related. Typically, he based this contention upon the encyclopedic nature of the writings of Maimonides (along with other medievals, to be sure). The central point is well taken. Jewish literature and history are fundamenally inter-textual and inter-contextual, respectively. Similarly, the the lines between genres of writings that Jews created, and between the Jewish and Gentile societies that they inhabited, were eminently permeable. While a person certainly needs to specialize, he equally needs to acquire as wide an awareness of the broader historical and intellectual context of his subject. Moreover, if one writes about Rashi and Ramban, for example, that should require learning the basics of medieval Hebrew poetry (and, or, piyyut). Or, perhaps, a dip into contemporary French and Catalan poetry too.
This lesson was driven home to me recently when I ended up finding something (not yet published) of significant Halakhic and historucal significance, in a collection of medieval poems. I was able to read and crack the poems because I spent a year studying medieval Hebrew poetry with the unforgettable, Prof. Yisrael Levin (a recent israel Prize winner), when he was on sabbatical in Boston. Literacy, yes. Specialization, not necessarily. It is enough, though, to undertake a study of halakhic implications in the poems of certain medievals. On the other hand, the article would have been rejected as a thesis topic because of כלך לך אצל נגעים ואהלות.
And that, in fact, is why I described myself and many of my American compatriots, as 'misfits.' It's not just a question of responsible scholarship obliging one to 'think out of the box.' It's that we take such broad view, intellectual history as a given. (In a sense, it's an academic חומרא). Unfortunately, far too many of our colleagues here don't really understand what we do, never mind the methodologies we employ (e.g. History of Ideas). Instead, because we are a bit less centered upon manuscripts and archives, we can be made to feel, well, inferior and dismissed as such.