Open tunings would surely benefit from a capo for key changes.
But I was referring to "open strings" in standard tuning. For example, play a standard first-position G chord, there are open strings. But what if you're playing in Ab and you want the same open string effect you get with G? You can only achieve Ab (in standard tuning) by using a moveable shape or barre chord, and that eliminates the open strings. Solution: Capo 1, play a standard first-position G chord. Viola. You get the same open string feel of the G chord but you're playing Ab. Not a cheat, not a crutch, it's more a tone/voicing choice. I can play moveable shapes all over the fingerboard, every chord know to man. Sometimes, for voicing purposes, I choose a capo and open-noted chords instead.