Discriminating against another person based on their appearance IS a bad thing. No matter how different another human may appear, they really are just like you. They think like you, they act like you and they probably have the same wants and needs as you. When it comes to humans the only real differences that make any impact are ideological and cultural, and in the context of those two things it doesn't really matter what you look like. An Asian person can easily talk and act more "American" then you, and a white person raised in any Asian land might talk and act like them. We're all human, and our differences are superficial 'r-a-i-n-b-o-w'

The Prawns from district 9 were most definitely discriminated against, and I know the whole situation was supposed to be an allegory for something that happened/is taking place to some people. I know why this trope is used in sci-fi, to. Racism is such a tricky subject for us, it's easier to present the situation to people from a more outside perspective, and make the "people" getting mistreated a different species entirely. Let me make this clear: I hate that. It's a trope that bothers me down to my core, mainly because it's incredibly stupid and even dangerous to think like that. Just because you can talk to something doesn't mean your differences are superficial, or ignorable. Different species are not the same thing as different races, and I feel that implying they are is insulting to both.

That's not to say I think other species should be mistreated, no, I am simply saying they make poor allegories for other races or real groups of people. Take for example the prawns again. Can you imagine what would happen if the humans didn't discriminate against them with our "evil racism"? Realistically, what would happen if they just allowed them to scatter into the city and live amongst everyone else? Towards the end of the movie we see a group of prawns eat a person who's severely injured... They ate him. Now imagine that happening in front of your little girl after a car accident takes place in front of your house. Or worse, imagine that happening to your little girl after she hurts herself. Now don't get me wrong, I love Prawns, but I am not going to pretend prawns and humans are two species that could live together. That's as much the fault of humans as it is prawns; neither one of us are "bad", we're both just too different to co-exist in any comfortable or functioning sense. Knowing that, it is hilariously tragic to know prawns might be used to represent ANY group of real people.

That's why I hate this trope. It really only works with Star trek aliens; not well written/designed aliens. There might be a damned good reason to treat an alien different from the way we would treat a human person, especially if our own safety is perpetually in question. In the context of intelligent, sentient things, that would be considered discrimination... but sometimes that could be a necessity. I certainly wouldn't let a synx into my house, regardless of how fancy it dressed or well spoken it was. Call me an ass if you want. =I

If synxes were real and people treated them the same way as they do other humans, it would probably turn out like this:
http://www.youtube.com/watch?v=gWycuaWJFCM