I'm in the "inconsistent/compromises grip" camp on this one. If all you do is punch paper at a slow rate, fine, maybe it will work for you *IF* you can get some sort of reference and repeat the same grip and the same contact with the same pressure with the finger each and every time. Good luck with that. Accurate shooting is all about stability, consistency, and repeatability. The finger on trigger gaurd simply isn't stabil under recoil. It would be very hard, if not impossible to be anywhere near consistent with it from shot to shot, especially during rapid fire or firing under stress. Lastly, repeatability would be questionable unless you totally adjust your grip between shots every time which doesn't work for stress shooting or combat/self defense shooting.
Don't use the finger on trigger guard as a crutch to avoid working out proper grip. You'll only be cheating yourself. The majority of the pros abandoned that technique for a reason.
