The reason to use a capo is if you want the sounds of open strings. Also, to use it to compliment another guitar playing in a lower key. To use it just to change to a key you can sing in brings up the other name for a capo (the cheater).

It's pretty useless when playing jazz. You want those punchy closed-string chords.

The capo used as a device to press two or three strings in alternate tunings is interesting, though. The Kayser seems to do this well. Shubb won't work. The G7 might.

Frank