Some Eurasians like to be proud of the fact that people can't guess our race. We believe it frees us from the stereotypes of being associated with one or the other of our ancestral peoples. Unfortunately, having an indeterminate ethnic appearance is likely to be an evolutionary disadvantage.
Most scientists believe that altruism towards our relatives is rooted entirely in self-interest: If I am more closely related to Jon than to Bob, then helping out Jon indirectly helps some of my own genes get passed on. So given a choice between lending money to Jon or Bob so he can pay the rent, or saving Jon or Bob from a burning building, and assuming no other complications (e.g. you're not in love with Bob, you don't wanna knock off Jon to get a bigger part of Grandma's estate), the average guy will choose Jon every time.
But how do we know who our relatives are, aside from Mom and Dad taking us to family reunions and letting us get our cheeks pinched by every one of them? The ways of determining people to whom we are likely more genetically related are known as "Kin recognition mechanisms" and are described in some depth here:
Kin recognition mechanisms use appearance, among other factors, to divide the world into ingroup members and outgroup members. This is not just ignorant redneck racism. It's genetic self-interest. Of course, looking at appearance is just the first stage of a mechanism which takes into account accent/dialect, body language, personal mannerisms, culture, etc. However, note that when they have nothing else to go on, and are dealing with complete strangers, people are more inclined to trust those who look more like them:
This doesn't and shouldn't mean all that much in personal friendships - we shouldn't let this influence our culture, our ideals, and the people with whom we associate; on the contrary, you should pursue whichever culture you feel more comfortable with regardless of your physically resembling its members or not. But in anonymous situations (customer service, business deals, dropping your wallet on the street), it's likely that both Europeans and Asians will tag you as an outgroup member and discriminate against you in favor of someone they perceive as an ingroup member.
Worse yet, there's not all that much corresponding benefit from Arabs, Italians, Latinos, etc. mistaking us for ingroup members and being altruistic towards us, because we'll trip up on the later stages of their kin-recognition mechanisms.
Now, we'd like to think that education overcomes all such racism and race-based feelings and implants a general societal altruism into everyone. Unfortunately, that's not much consolation to this Eurasian guy, jumped by a pack of Asian men because they saw him with an Asian girl late at night and assumed he was white:
http://www.mavinmag.com/v3_hate.html (cached on Google because original link is broken)
Why do men do such things? Why does an Asian man seem specifically angry to see an Asian woman with a white man, rather than generally angry to see any pretty girl with a man who is not himself?. This isn't just poor loser syndrome or a problem of Evil White Male Media Stereotypes. This animal behavior, like most animal behavior, can also be rooted in genetics. See this study, which points out that men worry not just about all males, but specifically about outgroup males cheating with ingroup women, because the women have a genetic advantage to do so, and if the men don't detect it, they'll end up devoting resources to raising a child who is the son of an outgroup male and is thus genetically less related to them than their own children:
So men get suspicious specifically to see an ingroup woman (one whom we think we recognize as distant kin based on her appearance) even hanging around an outgroup man. We're genetically wired to assume she has an ingroup boyfriend somewhere whom she's cheating on, and we think we're doing him a favor by helping him out and driving away the outgroup male from his woman.
In short, having an indeterminate ethnic appearance, regardless of any other social benefits, appears to be a net evolutionary disadvantage.