Forget for a second the fact that fair is a matter of perspective and almost completely subjective. Fair is completely irrelevant in the question. All that matters is what increases government revenues in a sustainable way.
Let's assume for the moment that the rich (defined as the 250k+ a year) are rich because they are better at investing. So, let's say that the national average ROI is 20%, but for the rich it's 25% . I don't think this is a ridiculous claim, but if you disagree then the rest of this won't follow.
Now, consider 2 situations:
A) Both parties have a 10% income tax (leaving 10 and 15% ROI). The rich guy start with 10mil and in 10 years they have 40.5 mil, whereas the average american would only have 25.9 mil. At year 1 the rich guy is putting in 100k and the average guy is putting in 100k (200k revenue). At year 10 the rich guys is putting in 405k and the poor guy is putting in 259k (660k). Because they are more able to increase their own revenue they are also more able to increase the government tax revenue.
B) The rich guy has a 15% income tax and the avg joe has a 10% tax (giving both parties a 10% ROI). At year 1 the rich guy is putting in 150k and the average guy is putting in 100k, which is a higher initial revenue. But with the increased tax burden the rich guys ROI reduces to 10%, therefore at year 10 he also only has 25.9 mil, and is paying 389k, giving a total tax revenue of 648k.
From year 10 onwards scenario A increases it's tax revenue at a faster rate than situation B. In this idealized situation Scenario A is more profitable for the government.
This isn't an argument for a flat tax though, there are a lot of points in between A and B that allow a marginal tax that are actually more profitable than A, and (more importantly) this is an idealized scenario that doesn't represent the real world in any granularity beyond the 2 extremes given, which are used to formulate the theory:
If the rich are more capable of increasing wealth than the average american then excessive tax burdens on them will actually decrease government revenue in the long term.
So, there's the theory, but the devil is in the details.
If the rich are rich, not because they actually generate wealth at a faster rate than the average american, but because they use unethical practices (for example if they steal it, say like Enron), then the above theory completely falls apart. In fact unethical practices actually destroys value, therefore it is actually dangerous for them to get more wealth (they would reduce the government revenue in the long run).
The above example is the view a LOT of people have on the rich, that they got there by cheating. I think that for some this is most definitely the case. I have met people like this before and it makes my skin crawl to see someone earn money for destroying value. That said I think it is a gross generalization to argue that this is the state of the majority of the wealthy, or even a significant minority.
The question is complicated (hey its economics what do you want) and both the democrats and republicans choose to simplify it to the initial question of fairness, because hey, that's an argument that sells. That's easy. When you're arguing with your buddy from Green Bay about whether or not your 49ers will take them down this sunday you don't bust out your stats sheets and start doing rigorous analyses of the teams. No, you say that the 49ers will win because Green Bay sucks and because Brett Favre is a creeper.
Anyways, I guess I haven't actually said much in this wall of text so I'll actually add my view of how things should really go:
The theory above rides on ROI being a kind of acceleration/momentum thing (as Einstein said, compound interest is the strongest force in the universe). In times of recession like right now, even with no taxes, there is not enough momentum in the base economy to allow for high ROIs from even savvy investers. Therefore an increase in taxes doesn't significantly change the long term outlook as these periods are a wash for most people. However in the "good times" there is a lot of base momentum in the economy, and at that time the wealthy have the ability to maximise their ROI, which means that the government needs to severely back off of taxation.