I like the idea, since it has proven 100% reliable in my G23 (so eliminates the reliability issues I've had with rail-mounted lights). For the downside, once you've shot your sixteen, your light goes with the empty magazine. Of course, unless you are in Mogadishu or live in the giant Gore mansion during a zombie attack, it probably won't be an issue.
I'm not a huge fan for a few reasons. It puts the weight at the rear of the weapon which gives the weapon more felt recoil as opposed to the rail mounted lights which reduces it. It could increase your chance of having a mag failure and seeing your rounds bouncing all over the floor. Once you reload, you do not have a light anymore.
I prefer to hold my Surefire G2LED or my E2e in my support hand.
Sorry, I don't mean to rain on your parade. If you like the set up, that is all that matters.