The Unforgiving Minute has a post on the dangers of magazine safeties.
Referring to the SR9:
It is the worst of the worst! With the magazine removed, the trigger functions normally, dropping the hammer, but the gun is prevented from firing. (…) It simply blocks the firing pin! Upon hearing a “click,” instead of a “bang,” the shooter knows little, because the pistol has told him little. Chamber might be empty. Might be a dud round. Magazine may be unlocked.
Being a Glock fan, I personally think the less external safeties the better. As mentioned in the article, a magazine safety should not be the only thing that separates your kid from firing your pistols.