When I was in graduate school, there was no discussion about how one's choice of research topic would affect your marketability. The general atmosphere at most programs is that marketability is a bad word, that you need to focus on being the best scholar possible, that you need to pursue your intellectual passion with no thought as to how your findings will be received.
I am swayed by this argument but I do think graduate students need to give some thought about how their topics will be received by other scholars. While you shouldn't change topics to match market demands (as no one knows what the market will be in 10 years or what topics will be hot), you should at least think about the intellectual "spin" that you choose to attach to it.
Chances are, you'll be hired by a department where you will be the only person who does work in your geographic or topical area. How can you explain your work so that a meso-american archaeologist or a middle-east religions person finds it interesting? These are the same questions you'll have to answer when writing major grant proposals, so it behooves you to keep them constantly in mind.
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Couldn't agree more. In my experience (at a UK university) this sort of advice does enter into the picture during doctoral research, but it might not be a bad idea to start introducing students earlier on to the perils (or joys, depending on your point of view) of choosing a very unusual/unmarketable topic.