I don't see the point of "happy go lucky, all are equal and nice to each other" settings either. My defintion of "melting pot" simply does not cover ghettos, racial quarters, and similar divisions.
So, it just seems to be that you're using a different definition for melting pot then Jürgen?
Basically, there are 3 concepts:
1) Isolated Communities with only one race and few or no outsiders.
2) Community consisting of several races, but each keep mostly to each other, possibilities for tension.
3) Fully integrated community of several races, no "ghettos" or tension
I think melting pot allows 2 and 3 in most definitions. The USA and especially big cities (say, New York) in the US are often considered "melting pots", despite the fact that there are also a lot of ghettos or even ethnic tension. Version 3 is certainly the ideal we hope to achieve.
...
Fantasy Races are humans with funny ears _and_ a specific culture. If you remove the "ethnic" differences between them, all that remains are the ears, of course. But on another level, you also tell us something about the state of the country/city/region - these races lived so long together that their cultural differences became irrelevant.
The 4E implied PoL setting seems to support this idea to some extent - Dragonborn and Tiefling seem to be common and accepted enough to live and travel to cities and villages. How could this be? My guess is that the past Empire already united all these races, and they worked together for some time, long enough for peaceful relations. But of course, that's not the only approach to PoLs (in the end, the reason why 4E uses these assumptions is because they work best for the game - the players can really see each "Point of Light" as a save haven, regardless of race or culture. If you wanted, each isolated Point of Light could also be "single-race", extremely skeptical to any traveller, especially one with horns, scales or pointy ears - unless all members of that community do have horns, scales or pointy ears, in which case, it looks the other way around.)
On a more general level, I think each appraoch has its merits, and there is also a good reason to use both.
You could have a "free and open" country where communities mix a lot of races. That might be appropriate for a recently colonized region, or just a country that came from uniting several races.
You can have an isolated country where only one race is common place.
You could have a country that was forcefully united, and now races that don't necessarily like each other have to work with each other in cities.
You could have a city that became a safe haven for all kinds of races after their homes had been threatened or destroyed, but the original inhabitants don't really know what to do with all these immigrants, and the immigrants prefer to stay among themselves, too.