You know chucky, you're somewhat of a pompus ass, aren't you?
The majority of engine failures are partial power failures. No, a fouled plug doesn't necessarily constitute an engine failure...but you are probably bright enough to know that one doesn't always know if it's a fouled plug. You're going to advertise to the new private pilot on a private pilot's forum that one should make such a stupid assumption?
I've had one or two plugs out of 28 on an engine cause enough vibration that it needed to be shut down, and many other times when a simple mag change was enough. I've also had engines vibrating hard that felt very much like a fouled plug, but involved lifting heads, fuel flow fluctuation (and subsequent failure) in a 206, vibration due to propeller malfunctions, and other problems which one could be forgiven for believing were simply a fouled plug.
You're going to sit there and waffle on about your blessed experiences and suggest that each private pilot out there should dumbly make the assumption that vibration or other symptoms of a sick engine are merely a fouled plug?
I experienced a manifold pressure loss in an R2600, years ago; the engine ran like a top, but nothing more than barometric pressure. The clutch failed, and though we had no other indications, the engine was full of metal. Certainly mechanical problems warrant getting back on the ground soon enough. Certainly an engine doesn't have to produce no power to be a failed engine. If it's failing to function properly, it's failed. A partial failure is as good as a total one.
The TPE-331 I learned, can experience an oil loss and run for a half hour. In your little heaven, perhaps an engine flying around without oil isn't a failure. In my world, it resulted in a forced landing on a hillside during a forest fire. Perhaps you haven't had the benifit of that experience.