I'm a fan of having a good quality light on any firearms that might have to work to earn their keep, but I find that the light often gets in the way of how I'd prefer to grasp the fore end. Also, adding the (admittedly little) weight a light sometimes messes with the balance of the rifle, especially if it is being hung off the end of a long fore end on a relatively lightweight build.
Have any of you seen tac lights with the battery separate from the head? Moving the battery to the rear of the handguards could both keep the rifle more balanced and make it easier to mount the light without having it get in the way. There are LED head lamps that position the battery pack at the rear and one of these could be modified to attach to a rifle but these lights tend to be more "flood" than "spot." They are generally not up there in the 1000 lumen range and most don't look like they'd hold up well mounted on a firearm.