If you are using the flashlight to search around, probably. The way I think of it though is this. If I'm walking around the house, gun drawn, lights off, its because I am pretty damn sure someone who doesn't belong in my house in there. If that's the case, I've probably got a phone in the other hand. I don't want to sacrifice the phone for the light. Also, I'm pretty crappy shooting with a flashlight in one hand. I continue to practice but I'm still better with two hands on the gun.
Additionally, remember that you don't necessarily need to point a flashlight directly at what you want to illuminate. You could still walk a room, muzzle down, and the light would brighten things enough to identify a figure. Like anything else, you need to train with the equipment to be good with it. Personally, I see a lot of potential failures with the separate flashlight. Its just a question of which you want to accept.
Personally, I'd rather take the risk with the flashlight attached and rely on good trigger discipline than search for TWO objects in the dark and operate TWO devices with BOTH hands seconds after I've been woken from deep sleep.