I hate this cliche:
"The only safety you need is the one between your ears."
But let's face it, whether you gun has a manual safety or not, your gun shouldn't be pointed at anything you don't want to shoot, and your finger shouldn't be touching the trigger unless you're ready to fire.
I've seen several guns in my lifetime (from BB guns, Ruger 10/22's, and 1911's) that could be fired with the safety on safe. The fact that the Glock doesn't have a manual safety doesn't make it more dangerous than one that does. One could easily make the argument that a manual safety gives you a false sense of security, whereas with a Glock, you're well aware of the fact that there is no manual safety and you're going to be extra cautious. I don't know if I'd make that argument, but I've seem some who have.
Never rely on a mechanical safety device. If you do, it's going to bite you some day.