I can see both sides of this discussion, and I'll say first off that I have absolutely no empirical experience with the things like you guys do so the relative number of good ones to bad ones is beyond my personal experience by a long shot. Secondly, the only Latin that I know is caveat emptor.
Heat related probelms with high tension coils have been around for a long time. These intermittent problems usually manifest themselves as shorted windings in the primary (low voltage, points/module side) due to insulation failure. In many cases, the short manifests itself only after the coil is hot because of expansion.
Perhaps an explanation of how coils work would be of interest here. If it isn't, just flame away!
It's important to understand that the primary is a high current, relatively low voltage environment. I say relatively low voltage because, using a simple breaker point analogy, when the points close, low voltage (from the battery, mag or flywheel mounted charge coils), flows through the primary. This is NOT when the spark jumps the gap. What is happening is that energy is being put into the coil, it's rate determined by the inductance of the primary and the voltage impressed across it. As the points close, the current through the primary will start a zero and linearly ramp up until the points open. Powdered ferrite or a physically gapped core is used here; the energy is actually stored in the physical gap or distributed gap in the ferrite material. When the points open up, this energy has to go somewhere as now you've opened the primary circuit. What happens now is that the coil "unloads" and reverses polarity, sort of like a spring being expanded, then released. The voltage across the primary goes from, say, +6V to -100's of volts during the open circuit condition. This is what they call "back EMF, and it's the same principle as how an electric motor spinning is just like a generator. Because the primary inductance has provided many times the coil charging voltage, the secondary-to-primary turns ratio can be reduced, making a more efficient coil. Peak primary currents in ignition coils are in the neighborhood of maybe 10 Amps. The condenser in parallel with the points limits the peak voltage across them to help keep them from pitting. It acts as a shock absorber to keep arcing down. It also serves to limit the peak voltage across the primary and lengthen the discharge time across the spark plug.
Electronic ignition modules do the same thing as the points, but with a transistor. In this case, the transistor is a switch that's either on (points closed) or off (points closed). The electronics that drive this output transistor can be made to do many things better than a points ignition, such as tayloring the discharge and advancing the timing. 99.9% of the failures of electronic modules in this regard are shorting of the output transistors that drive the coil primary.
Because of the high currents involved in the primary circuit, good grounding and connections are mandatory. A poorly connected electronic module may fail permanently for this reason alone. If a connection is bad on a breaker points ignition, nothing bad happens. If an electronic module isn't connected well, you might blow it.
When the secondary (high voltage, spark plug side) goes, the coil usually ceases to work forever due to carbon tracking. When a secondary looses it's insulation integrity, the high voltage usually arcs across and forms a carbon bridge that effectively shorts out a portion of the secondary. At this time, the heat generated from the arc melts the insulation off adjacent windings, which in turn shorts out more, etc. etc. Secondary DC resistances are usually in the 10's of thousands of Ohms. Carbon tracks are not dead shorts and may be many thousands of Ohms themslves, so "Ohming" a secondary or even hi-potting it may not offer conclusive evidence of failure.
Sparks are interesting things in their own right. We all know what resistance is; well how about a negative resistance? If one increases the voltage across a resistor, the current also increases. If one increases the voltage across a spark plug's gap, the current decreases. This is one principle that allows electronic ignitions to adapt themselves to different conditions inside the combustion chamber by using the current through the spark plug as a sense mechanism. But that's another story.
