Blender 2.7x has a Skeleton Sketching feature which I suspect is not used much.
//(mainly because we've had bugs in this area that weren't reported for years ... or at all, I found errors just looking over the code. eg: rB28f31bdfd807e35d7016623cc9af8467165a1052)//.
My impression is that riggers normally want to control where their bones are placed with greater accuracy then you'd get from a sketch.
I'd be interested in feedback from users, are people using this and finding it saves them time?