I use two snapgates and do so well aware of the 'dangers'. When I teach others, I talk them through the arguments below and let them make their own choice. When teaching my daughter, who was about 11 at the time, I did the same and was quietly pleased when she chose to use snapgates.
In my view the arguments boil down to the following:
For Screwgates (or screwgate on long only): Snapgates can undo all too easily when pushed against other rope, rock, or anchors. This is much more likely when falling or swinging, both of which happen readily on traverse lines and pitch heads. Unless you choose to use three cowstails (or equivalents), you will be reliant on just one Iran and this is often at the points where you are most likely to slip.
For Snapgates: Screwgates are a faff to do up and undo. This means it is either more likely you will slip at those tricky moments while you try desperately to undo them while at full stretch, or you won't do them up anyway. You won't die, but you'll get tired more quickly, and are more likely to get injured swinging around. Some way you can use a screwgate but choose to ignore the gate and have a screwgate anyway. However, it is incredibly easy for the finger to nudge the gate slightly when engaging the krab, particularly when at full stretch. This can quite easily lead to the gate not actually shutting making the krab scarily weak. Now this really is dangerous. Crucial to the overall argument too is that it is possible to be much more confident a snapgate won't undo by careful placement. If people are taught to think about what happens to the krab if they do fall, they can learn to clip them in so that the spine of the krab is the one that will hit nearby rope, rock, or anchor , totally removing the need to worry about whether or not there is a gate. So learning where to place the krab means you can use snapgates as safely as screwgates, but without the issue of the screwgate's gate moving a bit while clipping in leaving the gate open (which is dangerous).
(Note that Cap n Chris is right when he suggests Petzl Vertigo krabs as perhaps the ideal ... here there is no chance of the gate being held open as described above. They are more expensive and heavier, but I do keep thinking I might switch to one of these on my long and, as a result of this thread, might even get around to it, so thanks eveyone!)