Klaus, I agree strongly with what you've said here. If two microphones are the same make and model and ought to sound the same, a direct comparison is a nice, quick way to check that. If they're variations on the same basic thing (e.g. a KM 84 vs. a KM 140 or KM 184), sure. If you know something about one of the microphones, then after such a comparison you will know something about the other one, too.
But when two microphones have inherently different characteristics, your judgment that microphone "X" sounds better than microphone "Y" in setup "A" tells you nothing about which one will sound better in setup "B", let alone "C", "D" or "E". Your preference may be a total fluke--particularly if you weren't there when the recording was made. Different characteristics may come into play, e.g. differences in directional pattern in different frequency ranges.
--best regards