Yeah, but consider that some rare words are much stronger indicators of topic than more common ones. Even more so if you look at n-grams. If you use something like wordnet you can get a lot of meaning out of low-frequency words and throw away the meaningless higher-frequency ones that occur in too many categories to be useful.
Sure, there's value in rare words, but I don't think anything that occurs across the corpus fewer than 3 times is going to tell you anything useful. You need a certain amount just to have it be a real signal. What was the least frequent useful word in the data set, msalahi?
