"I'm wondering if [lasers] are useful in conceal carry or home defense..."
With a big enough battery the laser is all you need. "Bullets be damned!"
But seriously, I'm planning to have a laser and a flashlight on my Judge as a home defense weapon, but no laser on my carry weapon. I'll have the weapon loaded with two .410 rounds and 5 .45 LC's.
My assumption is that home defense will most likely take place in the dark or in close quarters. In neither situation am I or my wife likely to have the gun stretched out at arms length attempting to aim through the sights at the intruder. It will be down at my side or close to my chest to ensure it is not taken from me. The flashlight is to ensure the intruder is a bad guy, and the laser to to reduce the chances that I will miss and hit someone else.