Surprisingly once I got used to it it became more natural than normal shifting.
The main reason....is for racing purposes. If you're powering out of a corner and are still leaned way over....you can easily run out of gear before you're upright enough to get your toe under the shifter without causing any problems. With 'gp' shifting you can upshift while leaned over pretty far, and not risk stuffing your foot on the ground or disrupting the bike (aka hisiding into the wall).
I rode with normal shifting since I was 7, and only a year ago I switched to gp. I'll never go back, unless I get a bike that you absolutley can't switch over.
Oh and I've NEVER missed a second gear shift since going to gp!