I don't think I quite fully grasp the moral intuition behind the so-called "Buffett Rule" (a moniker so godawful that the only way I can mentally salvage it is to imagine tiny parrots mobbing a caricature of a man on a beach who just blew out his flip-flop after stepping on a pop-top). It seems to me that there are at least two components, and I'm not sure which one is more relevant to the morality of taxation. Note that in both cases, the warrant appears to be that taxes provide public goods where MB > MC. Questioning this assumption is ill-suited to the mission of EE, so I won't waste much time on its consideration.
The consequentialist reason:
The wealthy among us attach lower marginal value to income (or wealth). In terms of utility, they can afford to pay more in taxes.
The deontological reason:
Income inequality is bad for society. A high GINI coefficient, even devoid of context breeds social unrest. Poor people are demoralized and seething resentment grows, perhaps exacerbated by crass displays of idle wealth by the spoiled children of titans of commerce. Add to this an assumption that wealth is earned illegitimately, perhaps by coercion or by the rents generated by cronyism to bake a superb cake of "soak the rich".
I'm sure I'm missing plenty here. The fairness heuristic is frequently invoked, or social justice or simple indignance. This is all fine, all good and wonderful, but the EE hook is this feeling I think people have that wealth is often illegitimately earned.
And indeed, this is where I have much in the way of common cause with many of my more conventionally left-leaning friends. It's a fairly plain fact that lots of wealthy people have earned their fortunes by currying political favor rather than conducting euvoluntary exchanges. Not all, of course, but enough that I share some sentiments of pique. I propose however that the failing lies primarily in the metastasis of Congressional activity. Taxing the lampreys that cling to the flanks of leviathan cannot be done without poisoning the entire bay (how's that for a terrible metaphor?). Starving the beast of Big Government strikes me as a more appropriate policy response.
I think the EE questions are in yonder clam beds: just how much of America's accumulated wealth was generated by non-EE methods? Is the answer to this question relevant to tax policy? To Constitutional limits on the power of the Congress? Is it morally appropriate to treat non-EE political-rent-holders the same as euvoluntary market participants? Who should be held accountable for the scope of the corporate welfare-warfare state? Do the incentive effects of taxation change depending on the strength of a firm's presence on K Street?
By the by, the reason I make so many nautical references (since nobody asked anyway) is because I'm a former Sailor.