Yes acidental discharge can happen and very rare. This is why proper handling is very important. Guns like Glocks are good open carry guns Ina external holster. I conceal carry a S&W Bodygaurd 380 and its a double action so no safety is ok although it does have one. The striker fire, safety action and other non double action should always have a safety IMO. These guns are not safe to conceal carry unless there is a safety. I also carry a S&W shield .40 which is a striker fire but has a safety. S&W offers their M&P line with and without a safety.
Most unwanted discharges are because someone is showing off their weapon. I get asked all,the time and simply say no.
Yeah, I open carry the Glock in a black hawk holster that is solid plastic has a "button" that must be pressed to release the weapon. It's perfectly safe in the holster... but it gets removed often enough.
I agree with your thoughts on conceal carry weapons... that extra layer of safety is nice, just have to practice/train with it in those conditions so that it's second nature when you need it.
Speaking of poor handling, one my clients so that I carry and wanted to show me his XDM 45 that he carries in his car, he hands it over and I of course do a safety check, it's cocked and loaded. Thanks, here is your gun back, please don't accidentally shoot me!