I already explained the rational behind "because they can afford to", but I'll summarize for you once again:
Every tax dollar paid by someone who is incredibly wealthy can represent one less tax dollar which must be paid by someone who can't even afford to pay their heating bill. While, say, $100 may mean next to nothing to the wealthy individual, it could mean the literal difference between life and death for someone who can now afford to eat, pay a utility bill, or whatever. So if a wealthy person is made to pay, again say $100 more in taxes, to relieve that burden from someone who is far less able to pay it, then it should be done.
Obviously not all situations will be this dire or dramatic, but the point is the same. Those with the means to pay should be paying more to ease the burden on those who have less means to pay. That's what paying your fair share is all about.
FWIW, I probably wouldn't have as much of a problem with the idea of raising taxes on the wealthy if it was presented with an argument of "because they can afford it more than anybody else".
*rolls eyes* That's exactly what I HAVE been arguing.
The fact that they can afford it more than anyone else is WHY it's part of them paying their fair share.
As for class warfare, I think the wealthy are constantly at war with the middle and lower classes, and they have the power to fight it on their terms by using wealth to keep control.