First of all, any speaker spec sheet that claims a frequency range that wide is highly suspect. There are a lot of ways to cheat at specsmanship, and I suspect this particular manufacturer is well-practiced in them (assuming you're citing a real case here).
Now, it's certainly possible that a speaker might be able to deliver some energy at those frequency extremes, but that's not what matters. What matters (to the extent that this spec matters) is how flat that response is--whether it's louder at some frequencies than others (which it always is) and by how much. That's why an honest FR spec will look like this: 48-18kHz +/-3 dB. In other words, no frequency is reproduced at a level more than 3 dB higher or lower than some "average."
Now, there are still a lot of ways to fudge even a FR spec like that, so it's important to take ANY spec with a large grain of salt. But at least a manufacturer who quotes a dB range in his FR spec is making a nod toward honesty.
I'd also echo Arthur's point that speakers usually are the weak point here. You can spend less than $300 on a receiver and CD player at Best Buy and get flatter FR than almost any high-end speaker system on the planet. (In fact, a lot of so-called high-end speakers aren't all that flat, because the manufacturers are trying to make their products sound distinctive.)
