My HD weapon has night sights, a weapon light and a Lasermax. I also have a handheld Surefire next to it. My HD plan calls for the handheld to be used first if possible. The weaponlight and laser get activated only when I've verified that there truly is a threat - assuming I have the luxury of that much time. If I don't, both the weaponlight and the Lasermax can be activated with a flick of the trigger finger.
To me, there are two downsides to a weapon-mounted light: one has already been stated - by definition, you're pointing your weapon exactly where your light is pointed. The other is that the weaponlight precisely pinpoints your location.
Honestly, I don't know the right answer. We, and the experts in this sort of thing, can argue all day over the "best" strategy, with upsides and downsides to everything. I say just have a plan that's safe for your circumstances, and then train with it.