I think it's obviously good to be able to illuminate a target in a dark environment prior to the commitment of pulling the trigger. Gotta know exactly what it is you're about to kill. What ever form that illumination comes from (hand held light, mounted light...whatever) that works for you, it's a good thing.
As for lasers...never saw a good marksman who owned one for defensive purposes. Not saying they have no value at all, but to me they are pointless. If a bad guy is close enough and threatening enough for me to put a laser dot on his chest, that guys worth putting a bullet hole through that same spot.
I'm not interested in brandishing a weapon to kill someone who doesn't deserve it, but if you break in to my home, I am not at all interested in the "wonder if a laser dot will do the trick" experiment. I know exactly where the bullet will go, don't give a rats ass if the bad guy does or not....he'll find out soon enough.