On one hand, if you don't level match, and there is a dramatic difference between cables, then that also says something about the cables.
Yes, it says that they have different impedances.
The fact that bulk electrical differences in cables can affect the sound is utterly uncontroversial. The longer a cable, the more resistance it's going to have. On the other hand, length isn't the only
factor: the larger the cross-sectional area, the less
resistance it's going to have. So given two different cable brands you can't just assume that you can cut them the same length and expect them to have the same impact on the electrical signal.
The reason that we objectivists insist on a level match is that differences in volume alone are often misinterpreted as more subjective quality differences. Sometimes the louder one seems to sound better when it really just sounds, well, louder. So it really isn't that informative if differences that are audible before a level match go away after
a level match. After all, once the test is done, you're going to adjust your volume level to whatever subjectively suits you and the level differences will stop mattering.
So frankly I see no "pro" whatsoever about skipping the level match.