Why is aversion to skunk odor so universal among animals?

Skunk oil is not dangerous, so there would not seem to be any evolutionary advantage to being repelled by it. Dogs, for example, are willing to stick their noses in some rather unpleasant stuff, but even they learn to respect skunks. Does the odor resemble something that is dangerous? If that were the case then skunks would be at a disadvantage, assuming they are not put off by their own odor. Are skunks the only family of animals to use odor as a defense? Would trace amounts of skunk oil be effective in keeping deer out of gardens?