The manual safety argument is a circular one - if you're smart enough to practice trigger discipline, you're smart enough to use a manual safety. Likewise, if you're too stupid to practice trigger discipline, then a manual safety isn't going to save you.
My gut feeling is that adding a manual safety to a glock might make it a small bit "safer", but I have no hard facts to back up that argument, so that's NOT what I'm arguing
What am I arguing?
Simple - Don't argue with logical fallacies.
Some "bad" arguments:
"The best safety is between your ears" - That's true with every firearm, whether they have a manual safety or not.
"Keep your booger picker out of the trigger guard until you're ready to fire" - See above.
"The glock has a manual safety, it's on the trigger" - No (sigh) that's not a manual safety.
Some "good" arguments:
"You probably want to choose a firearm that came with an external safety" - You must trust your firearm. Having to fundamentally alter it suggests a lack of trust.
"Try carrying a week with it cocked over an empty chamber to get comfortable with the idea" - See above.