The Outside Member

Why are we required to have someone from a different department on our dissertation committee?

I decided it was to have someone on your committee unrelated to your field, so that you would keep your dissertation accessible to people outside the department. In that vein, I decided I should contact the one person from outside my department I've taken a class with, asking if he knew of anyone in his department who'd be interested in sitting on a committee in my topic.

My advisor seems to think the "outside" requirement is just a formality, and I should find someone with similar interests. This does not appear to be possible. I've cruised the faculty listings on various related departments, and it just so happens that my topic is really only investigated in my department. No outside expertise is really relevant. There was one person who looked like he was interested in the topic...but that turned out to be an old website from 2002; the person has moved on, and the department hasn't updated its website yet.

I'm thinking my plan will prevail. The only other student in my area who has a committee picked the outside member as the only person outside the department she'd taken a class with, which is more or less my plan (except I want to use that professor to network, instead of outright asking).