...there's no good reason I've ever run across or thought of...
Well, what about someone who
inherits a Mac with a firmware password enabled? Or, perhaps more likely, what about someone who acquires one of the hundreds of fire-sale Macs that schools and other large organizations regularly dispose of when upgrading their equipment?
Such Macs are reasonably likely to have had firmware passwords enabled due to institutional security concerns, and it's not too much of a stretch to imagine those passwords sometimes not being disabled prior to disposal.
