From a practical perspective, many more bugs than are being marked as "normal" are actually "normal" by this standard. Most of the bugs in our database are probably "low" priority in the sense that we just can't get around to fixing all of them. I can see the argument for making the most common case the default (though I share others uneasiness with doing so).
A good thing is that if the default is normal and the bugmeister then lowers it, that gives a negative feedback to the reporter, whereas if it's as the default, it doesn't.
Most bugs that are bugs should be normal, but perhaps we don't have the resources to fix each of them.