The reason to use a laser sight for handguns, and the reason to use a Red Dot sight (for rifles) is to put the reference for the point of aim onto the same plane as the point of impact.
Let me try to state that in more clear terms: the laser dot sights for handguns accomplish the exact same thing that the red dot sights for rifles accomplish. Plenty of our combat troops use the red dot sights for their M4's, and plenty of our civilian forum members have red dot sights on their AR's, but some of those same people bad mouth the laser dot sights used on handguns. The devices accomplish the same goal, but since one of them (red dot sights on rifles) is battle-tested super cool it is "OK" and the other one (CT lasergrips) is seen as a "crutch" for people who don't know how to use iron sights on their handguns.
The big difference is that the red dot on the rifle (or a red dot on a pistol), although visually in the same plane as the target, is in reality only a few inches in front of your eye. It's always there whether it's "on" the target or not. If it's not "on" the target, you can still see where it's at and immiediatly move your barrel until the dot is 'on" the target. That's why you see red dots and not lasers on military weapons, shotguns, open class IPSC pistols, etc.
With the laser on the pistol (or rifle) the dot must physically strike your target to appear. If your point shooting skills are such that the laser dot strikes the target every time you raise your weapon, you have no need for a sight anyway!
In the real world, you'll find that many times your dot will not appear on the target and you must sweep your weapon around until it does. You will not initially know which way to sweep your weapon to get the dot on target and 50% of the time you will go the wrong way, making for a very slow shot.
Indoors it's usually not too bad, you can see the dot on the walls or furniture. Low light outdoor is usually OK also in a cluttered target area. But in any kind of daylight shooting situation, if the dot is not on your target then it's usally not in sight and you're reduced to randomly sweeping the gun until the dot appears on your target or on some background object to give you a reference to where it's at.
All this assumes you're shooting at a nice stationary piece of paper. Try getting the laser dot that you can't see onto a target that's running, swerving, ducking, etc.
Very few people train enough to effectively use one sighting system, much less one for low light conditions (laser) and another for daylight (sights). It just adds another decision point into your OODA loop (observe, orient, decide, and act) and slows you down if you have to make a decision about which sighting system to use.