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I don't know what the book has in mind, but your first point is the main reason I would want to filter out the high frequencies with passive analog components.

Low noise and low distortion are important in audio.  It makes a real difference when the amplifier has to still have low noise and distortion to 200 kHz instead of 20 kHz.  The former needs 10x the gain bandwidth product to do the same thing, for example.

We often forget that opamps and the like don't really have infinite gain, although the simplified opamp equations usually assume that.  The assumption is valid enough as long as the actual closed loop gain is significantly less than the amplifiers open loop gain.  The closed loop gain of an opamp circuit is:

     Gain = F / (1 + F/G)

where G is the open loop gain of the opamp and F is the fraction of the output fed back to the negative input (negative feedback).  As long as G >> F, the gain is close enough to F.

Usually we want G to be at least 10x F, which means the final gain can be from F to 0.91 F, or up to 2.8 dB below F.  An absolute error of 3 dB in an audio circuit is usually of little consequence since the overall volume is probably user-adjustable anyway.  

However, if the gain varies over frequency then it matters.  "Good" audio circuits are expected to have a flat gain within 3 dB over the 20 Hz to 20 kHz range.  The open loop gain of amplifiers can vary significantly over frequency.  For opamps, there is a single dominant pole at a low frequency.  The result is characterized as a minimum guaranteed gain⋅bandwidth product.  Another way to look at it is that the gain⋅bandwidth product is the frequency at which the gain drops to 1.

Let's say we have an opamp with 1 MHz gain⋅bandwidth.  To use our 10x rule of thumb, we want the gain at 20 kHz to be 10x the closed loop gain.  That means this opamp can't be used to amplify audio signals by more than 5x.  To get the same 5x and be able to handle frequencies up to 200 kHz without causing trouble, we'd need a 10 MHz gain⋅bandwidth opamp.  That's going to have more noise than the 1 MHz gain⋅bandwidth opamp when both are designed for low noise.

And, it isn't all about flat gain.  To handle higher frequencies, you need higher slew rate to keep the system linear.  If the system becomes non-linear, then those high frequencies your ears will ignore can cause noise components at frequencies you won't ignore.  They can also cause noise at even higher frequencies, which then cause even more artifacts at audible frequencies.

Consider the case where the amplifier is presented with a sine wave past its design point.  That could turn into a triangle wave, for example.  That means lots of high frequencies were created by the distortion.  Some of those frequencies will be high enough so that individual components don't act linearly anymore, causing artifacts that ultimately become audible noise.  It gets messy.