Many consequentialist arguments tell us we are morally wrong to spend money on luxuries, or even comparative luxuries, while people are dying because of a lack of basic needs. It may be unrealistic to expect people to actually give away all their “excess” wealth, but really, it is immoral not to.The question is interesting because I think it it's the wrong question in a fundamentally interesting way.
The implied premise is that one enjoys luxury at the direct expense of others, that economics is a zero-sum game. But above a certain level, luxury and poverty are relative, not absolute; the question in this sense reduces to: Should some have more while others have less? The answer to this question is not quite so obvious.
There is, of course, a level below which poverty is absolute (not relative to the wealth of others): The level at which basic physical needs—food, shelter, cleanliness, medical care—are not met. Any humanist will quickly answer that this level of poverty for even a single person is ethically intolerable. But we must ask the empirical, causal question: Does the consumption of some luxury—even (the smug, self-righteous tone notwithstanding) a commenter's Ducati motorcycle—actually cause others to live in this sort of absolute poverty?
For about twenty years now, our primary economic problem has been overproduction: We have too much wealth. The price of this material wealth has fallen below its cost. At least in monetary terms. But what does this money represent? Money fundamentally represents the value of human labor and, for much of humanity, the value of their labor has, in terms of the ability to produce material wealth, become negligible or entirely nonexistent because of automation and productive efficiency.
If ten million people can produce enough food for a nation of three hundred million, but a hundred million cannot produce anything of value to that ten million, then ethically neutral economics would have those hundred million starve. To feed those "bottom" hundred million requires setting the price of food to zero for everyone, entailing that the ten million receive nothing of value from anyone; why should they labor mightily to feed a nation and receive nothing in return?
There is no economic solution to this problem. Even if—obvious ethical considerations aside—we were to literally harvest their organs and use them for medical experiments, there's simply no way to extract enough value from the bottom hundred million to save more than a few tens of millions.
The problem is not that those consuming luxuries are taking away wealth or resources from those in desperate need. The fundamental problem is that we are so efficient that our current economic system, geared as it is to allocating human time, has "efficiently" allocated a third of the human race to starvation, as not worth the effort and expense of feeding.
The solution must be political, but even there our options are constrained by reality. We cannot simply let them starve: A hundred million starving people will arm themselves with sharpened sticks and overwhelm the rest. Iraq, a nation of ten million strangers, is fighting the U.S. Army to a standstill; what chance does the Army have against ten times as many, and those their friends, neighbors and family? Even the most brutal, authoritarian Soviet-style repression can last only so long, two or three generations, before it collapses of its own inefficiency. On the other hand, no one wants to end up with The Marching Morons.
To solve this problem, we're going to have to make changes not only to our fundamental economic system, but our political and ethical systems as well.