Despite the fact that it is easier to make full barres with a nylon-string guitar, it is not dead easy. Besides when you use your index finger to make a barre, that finger can't be used for other frets and your hand position is less free. So, in other words, a barre doesn't replace a capo.

There are good fingerpickers who tune their guitar down (say to open "C") and then capo up. Using the guitar un-capo-ed with light strings would produce a lot of buzzing in this case.

Maybelle Carter tended to play using "C shapes" (ie she fingered the chords as if she were in the key of "C") but she would tune down as much as three frets and then capo up to get, say "B". Doc Watson uses a capo when he plays with other instruments or sings. He calls the capo his "cheater".

Before you say these people are cheating, you have to know the rules!

Murray