I dont disagree that using a high end cap shouldn't need a bypass.
But I know that some high end caps, like Jupiter copper caps, tend to sound "dark" or "veiled," and bypassing it with a silver bypass cap helps to brighten the overall tone a bit, as well as increasing clarity.
It doesn't completely change the sound of the main cap but it definitely influences the tone.
Some high-end Foil+paper/oil/wax caps caps also tend to discharge more slowly than a good film/foil or metalized poly caps of the same value, So bypassing them with a 0.1uF cap from the same brand/line will maintain their sound, while also increasing their clarity.
That's what I did that with my NX-Studio crossovers, bypassing the massive Miflex copper cap with a much smaller 0.1uF Miflex copper caps and adding another copper bypass to the bass circuit.
You dont need to go crazy expensive on the bypass caps, especially since the value should generally remain 0.1uF or smaller.
With cap values over 50uF you can use a cap that is 1/100th the value of the main cap. (Ex: 100uF cap with 1uF -or smaller- bypass)
I recently had someone ask if they could go all copper caps on the midrange on a speaker, same as they wanted with their tweeter circuit. But that would have cost well over $10,000 in caps that would need to be bundled in parallel.
We suggested that a better alternative would just be using copper bypass caps instead.