Our arguments would apply without modification if the situation were reversed. It doesn't matter how much we like our candidate or dislike her opponent, winning the votes is winning the votes. Our formal foundation is the principle of enfranchisement.I have said this time and time again to my friends, my family, and anyone who will listen. If the shoe were on the other foot, if seating Florida and Michigan means Obama gets the nomination, my position would not change. Count the damn votes! Gawd, this was my position in 2000 with Florida. The fact that we had five members of the Supreme Court decide our president made me forever see George W. Bush as an illegitimate pretender. I don't want the Democratic Party doing the same thing!
Check out the comments as well as they discuss another undercurrent that is being overlooked.
There are rules to allow the punishment to be rescinded, but these attempts are being thwarted by Obama in order to not have to stand up in front of those voters and be judged. More than the initial ruling, this refusal to agree to any remedy that empowers the voters to express their political will is causing potential voters to say "No, I won't vote for him under these conditions." There, again, is the different nature of the opposition.
And, in the end, that is where legitimacy is grounded in the US. While voters can roll their eyes and dismiss the mud-slinging, they want the votes to count. The fact that Obama will not face the voters may be dismissed by his partisans (Teh Rulz! Teh Rulz!), but it is what the uncommitted voters or those who kinda-sorta favor Hillary will make of it that will affect the general election outcome.
Elections are decided by the margins, sometimes by a few hundred votes. If a few hundred Democrats in key swing states decide that they can't vote for Obama because he cheated to get the nomination, there goes the White House.