Besides the increases in possible complexity and cost, there is very little to be gained.
Humans that can hear anything above 20kHz are very rare, and above 22kHz nearly non-existing. So, unless your intention is to design a sound system for your dog, it is just wasted bandwidth.
The -3dB point, which is the standard used for bandwidth, does imply that there is appreciable attenuation of frequency components that we can actually hear. So having some extra bandwidth so that at 20kHz you have less than 1dB of attenuation can somehow be justified. But the ear response is not constant either. As you approach the high end, your hearing is less sensitive and might even have nulls in the response. And if you are not a teenager you are very likely not to have that range at all.
However you must keep in mind that a high-C (soprano C) is barely above 1kHz. The highest note in a piano is ~4.2kHz. That means that 20kHz is nearly 5 harmonics of the highest note. The only sounds that might have significant content at that high range would be percussive (noisy/impulsive) ones.
But what about the source material? Standard audio CDs and most musical material are sampled at 44kS/s (so as to allow for filter roll-off) that means that physically there are no frequency components above 22kHz, other than distortion and noise. And the needed reconstruction filtering would reduce this bandwidth by itself.
So, in conclusion:
- unless you are into high resolution or high definition audio, the source material has no content on that bandwidth. All you would do is amplify (inaudible) noise and distortion. Thus wasting power.
- higher cutoff frequencies can provide for a flatter response in the high end, but so would a higher-order system response.