Bandwidth above 20 kHz is important for sound quality. However it's not a question of whether humans can hear sounds above 20 kHz. Human ears can't and most people really only hear below 15-17 kHz anyway. Some people will argue that the humans still somehow can detect ultrasonics and will provide research that suggests this, but I haven't investigated this research personally and cannot speak authoritatively about it.
Where bandwidth above 20 kHz is important for audio is in phase shifts. When you roll off a signal, any signal, from any kind of rolloff, phase shifts occur beneath the rolled-off point. Human ears can EASILY detect those phase shifts in music, although some people are better at this then others, and it also takes some practice (like developing a taste for beer or wine).
The further out of the audible range for the rolloff (aka a "pole" in engineering terms), the lower the audible phase shift will be. This is the primary reason behind why a wide bandwidth is actually a useful spec for determining sound quality on paper. It's not the last word on SQ, by any means, but it is indicative of a good design.
Also, the shape of the "pole" siginificantly effects the amount of phase shift. In general, faster rolloffs have an exaggerated effect. For example, in speakers, 1st order slope crossovers tend to sound better than 4th order slopes. They are more musical, as the ear doesn't detect phase shift anomalies as much. This is one of the tradeoffs that needs to be made in speaker design... Sometimes higher slopes sound better because of the necessity of steeper slopes to control drivers that are forced to work close to the limits of their linear operating range.
