The number one concept to keep in mind in keeping your handgun in your control when walking around on this earth is weapon retention. Perhaps the fundamental bedrock of this is your holster. You need to be finely tuned to the reality that having a worn, ineffective, or unused holster could turn into a very bad situation for you in a New York minute.
An unpleasant fact is that a holster will not last forever. Consider it the Murphy's Law of those who carry a handgun. Leathers, treated so they may be, will wear out and stretch over time. Even durable synthetic materials such as nylon and plastic, which can last in landfills for centuries, can and most likely will, swell, split, and separate after enough use in the real world. This can leave you with a holster that can dance around on your belt, guns that can fall out, or even cause accidental discharges in which the holster catches the trigger.
In a recent case in New York, a handcuffed subject snatched the Glock of an escorting officer right out of his holster and shot a second officer in the foot with it. In an investigation into the incident, blame was placed on the holster.
It was somewhat worn," [Police Commissioner Raymond] Kelly said of the holster. "If you look at the top of the holster where you push your thumb to open it, that's where he exerted pressure. There is a rip there."
Fundamentally, your handgun needs to be secured inside the holster and the holster needs to be secured to your body. Generic one-size-fits-some holsters without any retention level often do not hold the handgun properly, which can lead to the gun wandering inside the fabric. This can lead to a weapon falling out on accident, like when standing and catching on the arm of a chair. Worse, this can led to a gun being stripped away easier in a scuffle with an opponent. Many inexpensive clip-on holsters can be literally ripped off the body by even a modest sized opponent using a little leverage.
Besides the above, there is the aspect that a wiggling gun is harder to get a good solid grip on in a hurry--like when you need it. Do you really have the luxury of giving up an extra second or two in potential gunfight fishing for your handgun?
(if you carry, be it concealed, open, or LE, do so in a good holster that fits your weapon and secures positively.)
The worst dang holster out there is the one that isn't used. Carrying a loose handgun, 'gangster style' stuck in a waistband or pocket with no holster securing it, is just dangerous. The mark of a professional over a dangerous amateur gun enthusiast in today's world is this method of carry. It can lead to lost guns, accidental discharges, and just a lot of bad press that the gun community doesn't need.
In a very well publicized case, a Florida man's night at the bowling alley was ruined when he caught a round in the leg. Besides the gunshot wound, he had to face the fact that the round came from his own legally carried gun, unsecured in his pocket. Talk about insult to injury. Overall, though, he was lucky that the round only hit himself. The last thing you ever want to be is in a wrongful death lawsuit.
Especially over something as simple as a good holster.