I have a CT on my 642 carry revolver.
I keep hearing people talking about the light can fail, maybe, but mine hasn't in over 10 years I've used it.
BUT, I didn't take the sights off when I added the CT. They are still there and I still use them.
The laser on my revolver doesn't have a switch, its activated by a button on the grip. It is a great tool for precision shooting with a <2 inch barrel w/such a short sight radius.
Since a huge majority of SD shooting are at less then 6 feet, most of my practice is point shooting anyway and I don't use either sight or laser.
Where I have used it, and found the laser invaluable is snakes. Tons of rattlers in NE Wyoming. The laser is perfect. Put the dot on the head and dead snake every time. I've heard it said that rattler are mesmerized by the red dot, I don't know if its true, but it sure seems that way.
Its handier then heck when your crawling through the sage and cactus on the prairie while sneaking up on antelope. The little revolver with the laser works much better at 2 feet the a rifle with a scope setting 2.5 inches above the bore. The laser dot behind the ear is effective for dispatching wounded critters also.
But like I said, the laser does not mean you don't have your conventional sights, my 642 still does and I use both, however, at distance laser shooting is more accurate.
But the best use of the laser, in my opinion, is in dry firing. You can snap away all day and not really know where the bullet will go when the hammer falls, where the bullet would hit if you were actually shooting. You do with the laser. Where the dot is when the hammer falls is where the bullet would be if you were actually shooting.
Hours every week of dry firing with the laser helps my shooting when I'm using the sights.
When I first started using the laser, I was told, to change the batteries when you change the batteries in your home smoke detector, (like when you change your clocks twice a year). I started out like that and since have went to changing the battery once a year. I've never had a battery (or any other failure) on my laser. Never heard of anyone doing it.
I like flashlights, but a flashlight on a small revolver that I pocket carry is not an option.
Its a tool like the conventional sights, but like any other tool, just because you have one, doesn't mean you throw your other tools away.