What are the benefits of nickel-plated chains? Bicycle chains with higher model numbers (and slightly higher prices) are often nickel-plated or half-nickel-plated. What are the benefits of nickel-plated chains over non-nickel-plated chains?

How durable is the plating--does it wear off, or do its benefits last for the life of the chain?
Nickel plated chains have this done for aesthetic purposes, and to provide corrosion resistance. They are also less susceptible to the chain binding itself up. 

When two metallic surfaces create friction against each other, they create small areas of abrasion (Much like a plaque in your arteries). Over time, these increase to the point where the chain can kink in spots. Nickel helps to stave this off.

The wearing may be slightly higher than with a non coated chain, but the difference to the lifespan of the cogs (cassette) is negligible. Chains should be replaced around the 2500km mark on a regular basis, and cassettes anywhere from 5000-10000km depending on many factors. You'll go through 2-3 chains per cassette, and usually I replace my cassette with every 3rd chain no matter what.

Dust/dirt/mud and poor maintenance will do more harm and accelerate wear faster than any plating on the chain will.