Here in Northern Alabama, the people that know how to drive in poor weather seem to be far outnumbered by the people that drive as if it's a clear blue day no matter the weather or road conditions. We don't get a lot of snow here, but I have lived in Illinois, Colorado, Utah, and New Hampshire, so I fancy I'm one of the outnumbered ones.
The primary problem with not cleaning off the roof of your car/van/whatever is the big iceberg chunks that calve off at random moments during your trip. These are distracting and scary for *other* drivers, who sometimes swerve (and thus may lose control) trying to avoid your cast-offs. It's hard to tell what they're made of, you see; if you're just streaming powder, you're merely a nuisance, but if a big chunk goes flying, you're a potential threat (even if it's *not* ice).
These people annoy me *almost* as much as the people who clean off a little windshield circle right in front of the driver then go driving. But not quite. Like I said, we don't get that much snow down here. What we do get two or three times a year, though, is a nasty ice storm, which makes the windshield thing much more common.
And this in a region where people tend to drive as I described above. Of course there are exceptions made for the heaviest of rainstorms (whew), but other than that... well, let's just say I feel that driving defensively (I think) has saved me a lot of repair bills (I think).
(Ever notice people will readily admit to poor memory or addiction to caffeine, for example, but seldom-to-never admit to driving poorly or having a poor sense of humor?)