Yes, I think TM quite got it. There is not much information about them apart from what was said here. Only to that fire thing,
Quote:
Sauron can put fire to his evil uses, as he can all things, but these Riders do not love it, and fear those who wield it.
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I think fire might represent here the cleansing power, something which gives light in the darkness, and also the heat: as an opposite to cold and death the Riders represent. It is just a speculation, however, can't think of much more.