My experience with the grips have been very simple - when I grip the barbell hard while rowing, I get a much greater biceps and forearm involvement (more with underhand than overhand). When I use my hands as hooks only, the biceps and forearm involvement isnt as much, and it still stays a back exercise.
The same goes for a bunch of other back exercises like pulldowns or seated rows. You can make them a bicep exercise by gripping hard. Or you can target lats better by thinking of your hands as hooks.
From Yates to Cordova, there are enough good successful bodybuilders who on occasions use an underhand grip and build an awesome back. (In fact, the article from Cordova I linked to a few posts earlier clearly states which sort of grips would suit what purpose) At the same time, from Arnold to Cutler, there have been enough lifters who swear by the double overhand grip. What every bodybuilder will stress though is to use your hands as hooks and try to feel the lats.