I think it would be more useful for delay effects to have an input gain control versus current effect mix control.
You could have smooth delay fade in and fade out with trails using input gain control on expression pedal.
For example if you would play reggae rythm and you want to bring a long delay only on the last note of the bar and continue the rythm. Don't know if i'm making myself clear here. You want delay to catch only the last note using expression pedal. Input gain would work great here, and now you can't do that with effect mix.