I see two "normal" failure modes:
1) the gap opens up too large, which requires too high ignition voltage to fire
2) wear on the electrodes (center or outside) creates a rounded surface, which with increasing gap, this can requirer very high voltage to fire that the ignition system can't handle
I've only observed two outright spark plug failures:
One was on a vehicle I owned (2001 Frontier supercharged), the ceramic insulator surrounding the center tip broke apart. The truck would have an intermittent misfire and when I pulled the plugs, that is what I found on one plug. I have no idea what happened to the ceramic, it probably got crushed by the pistons and spit out the exhaust. Changed plugs and engine worked fine with no more misfire.
The other was a good friend of mine that does absolutely zero maintenance to his vehicles other than random oil changes. He fixes things when they break, which hasn't been too terribly a bad approach for him, as his vehicles routinely go 200k+ miles. Sure increases your chance of a breakdown on the side of the road though. He brought his 2008 Jeep Commander (hemi with 16 plugs IIRC) to me w/ 100k miles on it, said the engine was misfiring. We pulled the plugs and the originals were copper plugs with a lifespan of typically 36k miles. The center electrode was basically non-existant-- it was sunken down so far into the ceramic insulator, you could hardly see it. You could see arc trace marks across the ceramic insulator where it had been firing. The side conductor looked like the tip of a nail-- it has worn mostly away.
Spark plugs will never be something that will give you horsepower-- but they can hobble an engine if you let them go WAY too long, or there's some mechanical failure with them like a broken insulator.