I think anyone should be able to work a DX as many times as he likes. This way one could amortize the cost of the station over a larger number of Qs
Joking aside I think one becomes a 'inconsiderate DX hog' when he causes QRM. If he's not causing QRM I could care less if Joe Smith makes a 100 contacts with VU7AG. I really don't believe the notion that a 'big gun' contact costs a little gun's chance of making a contact provided that the big gun didn't cause ANY QRM.
I'll go out on a limb to say that I don't think the leaderboard hurts anyone's chance of working an ATNO. If ANYTHING it helps motivate the smaller stations to improve their station and skills.
It's also unfortunate that big guns in general get a bad rap due to a few inconsiderate operators that fall into the 'big gun' and 'top of the HR' class. We all know who those people are. I honestly believe most big guns take a lot of pride in their station and operating habits. Some credit is due for those who operate responsibly regardless how many contacts they make.