If you're in a small field or working comparatively (or both, like I am), chances are, no one department or university will be enough to constitute a good committee. A lot of humanities specialisations have literally 5 people or fewer in the US working on them. For example, I'm interested in vernacular Egyptian poetry (though I'm likely not picking that as my dissertation topic), and there are 3 people I know of who focus on that genre; one of them is in the UK, and another in California... This is because there are huge expanses of Arabic (or insert any non-western-European language) literature that get entirely ignored in academia, and people tend to gravitate towards fields where some groundwork has already been done, so the trend is reproduced generation after generation (so there are like clusters of well-researched areas/genres/periods and big gaping holes in other specialisations).
Anyway, in small fields/uncommon languages/comp lit, it makes perfect sense to have at least one person for another uni on your committee. Of course, in that case, going to school in NYC is pretty convenient.