Blues players often use a capo at the 2nd fret, especially the old time ones. The guitars they originally played were joined to the body at the 12th fret, but along in the 30's they quit making those and the new ones were joined to the body at the 14th fret. There are two known pictures of Robert Johnson and the first one where he is wearing a suit and hat he has the first type guitar sans capo and the second picture in tee shirt and smoking a cigarette he has a capo at the second fret. Hard to play blues off of bar chords and having the 12th fret against the body of the guitar makes a natural stop so it's easy to find the octave, especially if you're whipping a slide down the neck. As the electric guitars came in and single note solo styles developed the capo was used less. The other thing I note is the capo gives you a lower action on the guitar which is sometimes an advantage.

Pete Gallatin
http://www.reverbnation.com/patiopete