Those are some very good points you brought up,
mtarzaim. And you used some really good examples. 
So in general, it was to get across the moral of discrimination and hatred. As for how significant it would be for Luffy to know about it, I don't think it is that significant.
Before Luffy knew about --> he had to beat up Hordy.
After Luffy knew about it --> he still has to beat up Hordy.
Hence, I did not understand the significance. I guess Oda wanted to get a message across, but in world, it does not seem to affect anything. They made it seem like it was such a huge revelation, too.