Supposedly the neutral grip pull-up at a bit less than shoulder-width is your strongest position for vertical pulling. At my gym, they only have parallel pull-up bars that are wider than shoulder-width and the triangle attachment which most people use for rows. This attachment has your hands about 6" away. Does anyone know how the very-close neutral grip affects what muscles are utilized in the chin-up?
I know you get a heck of a lot more biceps work going with the little V bar. In fact, it seems like bi's get most of the work. Not sure if it targets a dif area of the back though. I like the 'modified gymnast routine' at T-mag where these are done at the end of the reg pull-ups. Awesome back program. Read it if you haven't. It was a short topic article a long time ago.
Actually, I find that I'm virtually just as strong using a "regular" (i.e., hands pronated or supinated) grip as I am using a parallel grip - so long as the hand spacing is the same. Try it and see.