I agree with the catylitic explination, and no the platnum coil isn't "used up" in the reaction at all.

Plugs fail over time for other reasons such as:
- build up of carbon or burnt oil residue on the coil. Carbon being a result of the burning fuel and oil.
- build up of silicon deposits on the coil. Some fuel has anti-foaming additives that will eventually leave deposits on the coil. I've actually seen plugs that glow quite brightly when hooked to a battery, but as soon as the glow ignitor is removed, the engine dies. The cause was a fine layer of stuff on the coil that prevented enough methonol from getting to the platnum.
- melting. If you run the engine too hot or lean, it's not uncommon for the combustion temps to get too hot for the coil.
- other metal particles. Sometimes you can actually see little "beads" on the coil where metal has melted on. The metal might come from anywhere, but it's most common in new engines being broken in.
- dirt or other particles can sometimes get in the engine and literally just break up the coil.
- too much compression can sometimes slowly suck the coil right out of the plug, sometimes it's not slow at all.

On the other hand, if you are careful with your engine, you can have plugs last for years. I've had single plugs last for multiple seasons until I get stupid with the needle valve and kill it with an overly lean run or something.