You'd never* encounter a piece of classical music set to a faster tempo than that, which also contained tempos slower than (say) 40, where you'd want to denote

You'd never* encounter a piece of classical music set to a faster tempo than that, which also contained tempos slower than (say) 40, where you'd want to denote both extremes using quarter notes. Ultimately, it has to do with useful ranges of human ability and perception. By the same token, you can reasonably expect an article on clothing sizes to say something to the effect of "XXXS to XXXL", without needing to explain why the range doesn't go further. * Excepting, of course, the handful of joke scores or works of postmodern classical that would surely be summoned to this thread if I neglected to include this caveat.