Batteries to go dead, switches to break or user to forget and fumble with, another mechanical link to go wrong, Too much light to see the little dot in some cases.
In a self defense situation NOTHING beats instinctive shooting and practice, practice, practice. At almost 63 I now do have "seasoned" eyes, but if I couldn't see well enough to be able to shoot instinctively, I would fear that I would not be able to see well enough to identify the target as foe rather than friend!
Nonsense.
Nobody said you need to take the iron sights off of the handgun. They're still there, and they work just fine in light good enough to see them.
And if it's too bright for the lazer, you CAN see the open sights. If it's too dim for the open sights, you CAN see the lazer. In many lighting conditions, you can use either one.
A lazer is great for instinctive shooting practice. I can (with an unloaded gun) draw, point, and simply click the lazer switch to see where I'm pointing. After a bit of this, the gun points where I look without much thought or conscious "aiming" to it.
A lazer is great for dry-firing practice for trigger control. Simply activate the lazer, and watch to see if it moves when you squeeze the trigger.
The fact that it also acts as a low-light sighting device is a bonus, IMO.
I shot a rattlesnake last summer right after dusk one evening, when the light wasn't good enough to me to see the iron sights well enough to shoot at something as small as a snake's head. The lazer, aimed a half inch low and slightly right (to compensate for the close distance) solved the issue.
I shot a feral cat inside of a hen house recently, and couldn't see the sights in the dark of the henhouse very well, either. It was a dark, cloudy day, and the hen house doesn't let much light in. The lazer solved that problem, too.
And I've no doubt it would do the same if needed for SD.
In SD, instinctive shooting and practice, practice, practice are great, but if a lazer sight is added, it gets even better. It's an addition to the firearm that takes zero space, adds zero weight, and replaces nothing except the factory grips, while adding a lot to the firearms functionality.
On batteries:
I replace the batteries in mine about every two years. I test the batteries each day by bumping the switch as I holster my EDC gun each morning. It starts getting dim before it goes out, and I keep an extra set of 2032 batteries in the off-side grip panel.
If I notice the lazer getting dim, I pull the grips off, switch out the batteries, put the grips back on, verify POA with the iron sights ('cause I know where to aim with those, too), and I'm good to go again. The whole process takes about three minutes.
Next time I get to Wally-mart, I can pick up a spare package of batteries for it and then put them back in the off-side grip panel.
If you don't want one, then by all means don't get one, but don't bad-mouth them without having given them an honest evaluation.
Daryl