Most musical energy is between 100hz and 10khz, and movie sound might stretch that down to 30 Hz or so. Of course there is sound outside this band, but most of what defines the sound to the ear is well within this range. Whether a speaker really extends to 20khz is perhaps not as important as how it sounds at 500hz, where vocals, guitars, horns, and strings have a lot of their sound. Besides, a low quality implementation of a cheap tweeter may go to 20khz, but it will almost hurt you when it does. When you hear screechy female vocals, or cymbals that sound like white noise, you know the culprit.
Frequency response is hard to measure, in real world environments.
The best a manufacturer can do is measure their speaker in an anechoic chamber, which is perhaps the most dis-similar thing to your listening room. Even several paychecks worth of room treatment would not compare. So how a speaker sounds in your room, with your system is what really counts. Unfortunately, there is only one way to figure this out - put them in your room and listen to them. Try out speakers in your room, if at all possible.