(For some reason I couldn't post in this topic, or others in the last couple of days. But it looks like I can reply to your question Magoo).
The conditions in the combustion chamber change constantly during operation, this is due to the quantity of air/fuel mixtures, advance of the spark and load the engine is carrying. Our Webers (and FI as well) are delivering different amounts of fuel mixture because of throttle imputs, so if you were to "cut" the entire process at once (by shutting the engine off at speed), whatever mixture was combusting at that moment will deposit on the plugs. If you made any changes in the main circuit (air correctors, emulsion tubes, main jets), you'd be able to read the plugs and determine the condition (you'd also get a general idea by the way the car was running at those speeds, but thats by the seat-of-your-pants).
Now, if your plugs have 1XXXX's of mi on them, and you've been running the car too lean, or have other defects and the plugs are physically damaged (heat breaking off the insulator, build-ups, oil-fouling from old seals/rings), then you can't read them. New plugs would be recomended and then do a "plug-cut".