[ishitatsuyuki]

Analog distortion of signals also affects the phase. A digital filter might or might not compensate for it, but it's not always bad.

Also, human ears tend to be more sensitive to amplitude than to the phase. Especially if the same filter is applied on both channels, which leaves the phase difference unchanged.

[serverholic]

It's more than just human ear sensitivity. Phase shifting changes the relationship between frequencies in a sound.

For example, phase shifting can change the amplitude of a sound. Analog hardware is especially sensitive to changes in amplitude so you might be introducing distortion just by shifting phase.