This has been discussed a few times. The main points are that the United Planets stands for good races. If you are powerfull enough you can leave this but no one will be impressed, if you are evil and dont hold a majority vote expect to have them set laws which hurt you. Ohh also, there are events like uprising where evil races lose most of their planets

not good news!
Also expect no help if you are an evil race, you will find no allies really, and those who do help will not be very trustworty! The bonus you get for the planet things are not all that massive usually and in most cases i try to do the right thing.
I think its great, like real life doing the right thing is hard, being evil is easier...
Luke :Vader... Is the dark side stronger?
Yoda: No, no, no.
Quicker, easier, more seductive.
Luke: But how am I to know the good side from the bad?
Yoda: You will know... when you are calm, at peace, passive. A Jedi uses the Force for knowledge and defense, never for attack.
But beware of the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice.

wow, how come the quotes from the original starwars are so powerfull, yet the prequels were so weak dialog