The biggest advantage, as El Diablo pointed out, is when you accelerate out of a left-hander, and you have to upshift while leaned over. GP shift makes this much easier. It's more applicable on the racetrack than on the street, of course, but since I'm used to having my racebikes set up this way, I convert my street bikes over as well.
Also, when you are downshifting before a turn, GP shift helps keep thing settled a little. With street-shift, you tend to stomp on the lever as you downshift, and sometimes it can cause you to chirp the rear tire a little if you get too excited. With GP shift, the action is a little more deliberate, and it causes you to downshift more smoothly, taking your time...
The only problem GP shift causes is when you are entering a decreasing radius left hander, and you have to downshift in the middle of the turn. Turn 1 at Daytona is a perfect example. You can't do all of your downshifting before the turn, because you would overrev the bike. So you have no choice but to catch the shift after you're already leaned over quite a bit. That can be a pain in the ass! We have a turn at Firebird like that, too. But you learn to deal with it. The benefits outweigh the losses for most racers.
On the street, though, nyehh... I don't think it matters either way.
A fool and his money are soon partying.