We are still not all talking of the same thing. Steve, your tempo may be perfect and uniform, but your playing along to the metronome doesnt mean you are nailing the beat ON the beat. There will be a tiny accent (usually just behind).
BS. Not sure how you'ld set that metronome up in the 1st place. It would need to be perfectly in synch with at least the 1st part of the song. Concidering tape decks and other media dont all play at the same speeds either, its kind of hard to fix a stake in the ground.
I agree in principle with what you say, though Im less convinced it is quite as prevelant as you suggest. Any hint of ralentando will twart the metronome, so certain types of music are not suitable for this kind of tempo test. I'll keep it in mind though.