Guitarists tend to stick a capo wherever it will give them a pitch most comfortable for their voices, while still enabling them to play everything in the easy fingering of the instrument's home key. If they did what they expect we fiddle and mandolin players to do -actually learn to play in all those keys- they'd have a better understanding of what's involved, and might decide to develop their vocal ranges a bit, instead. Most don't seem prepared to make the effort, though (grumble).