but I think you've somehow sidestepped the purpose of such comparisons.

Actually, no. I was dismissing the purposes as irrelevant to anything truly useful. I understand that it's important to the corporation owners of these channels, and their shareholders, to see these figures. But if these figures represent a fight over an increasingly small share of the potential market for news and opinion -- as more and more people tune out from the bullshit and the claptrap -- what's actually important to note (but is ignored as a presumptively-unimportant matter when such figures are reported) is that the figures are actually irrelevant, the fight is irrelevant, and the companies and shareholders are trying desperately to live in a world where these ratings reflect anything of true importance.