Some issues I have with systems like that are -
1. Can you get a holster (mentioned before) - I have a rail light (it came with my XD package and the only holsters I can find have a rail on the holster for the light, but the light can't be on the gun in the holster;
2. Turn on the light and say "Hey bad guy here I am!" At least with a handheld light you can move the light away from your body and even point separate from gun;
3. With a rail mounted light you point the gun at everything you point the light at ... including your spouse and children

Another benefit of a flashlight over a rail light is that it gives you "knuckle reinforcement," a striking tool, and a really good flashlight you can use in any activity.
I'd recommend a crimson trace and a flash light over this arrangement.