I'm with you on that. I know some people, though, just love the "Grudge Match" aspect of seeing which of two powerful women is most powerful. And some of these same people don't want to see a man fight a woman, because when he hits her it is viscerally wrong to see happen, however lame that sounds in the context of a super battle. WW vs. Power Girl is interesting just because their outlooks and personalities are SOOO different. But again, you're right: the cheesecake factor is probably at pkay here. I think editors are afraid of alienating readers, quite frankly, by having Wonder Woman beat a man. I doubt that really would alienate modern teens, however - or 30-somethings, because of the superpower aspect. If you have the power, no matter your age, gender, etc. YOU WILL WIN. And it's more tense a situation if Wonder Wonman fights ,say, Hercules or Ares. They are men, these particular men are vicious fighters without a great deal of respect for their fellow humans; WW is committed to peace - through strength when needed, but not a first resort - and her feelings for those around her may intitially make her hold back.
Now that Donna has achieved her full powers and that it has been revealed that Donna is a magical mirror image of Diana come-to-life, Donna should be virtually as strong as Diana. Her height and weight should also be very similar.
I would be very interested in seeing how Donna would fare against Power Girl and Mary Marvel.