Religion is still necessary because humans are not naturally good, but actually naturally evil. We are always tempted to lie, cheat, steal, all to gain as much power as we can as fast as we can. Even your example of back scratching is based on power. If we were truly good, we would give out of the goodness of our hearts and expect nothing in return.
Now, a rational human may find that religion is unneeded, but the world is not full of rational people. I would bet that you will find a higher concentration of them here, which means that when you appeal to them, they will respond. But really, the irrational people of this world probably outnumber the rational, and therefore you would not have peace if you somehow did away completely with religion.
By the way, whenever I talk about a rational or irrational person, I mean someone who can or cannot tell why we should do good things and avoid bad things.
The rational human can see that, if humans work together and get along, we can achieve greatness for everyone. A rational human can see that if they do good things and don't do bad things, even though it may not immediately bring them power, it will eventually do so because it aids humanity as a whole and therefore that individual personally. But the irrational human, the majority I would bet, only thinks of themselves and their own immediate power. Which is where religion comes in. Religion tells us to do good and not to do evil, but instead of explaining reality, it makes up parable after parable to explain why we should do so. Interestingly, if we look at religion logically, we can see that many parts of it are irrational, namely the ideas about the supernatural. A place in the sky where you go when you die? A cosmic being that can do anything, that can cause seas to part and bread to multiply? These ideas violate rationality and reality, yet they have meaning for the irrational mind. Irrational reasons that influence irrational people. These stories teach people morals about right and wrong, that one should do right and not do wrong. These ideas are important to the irrational person, who without them may indeed cause harm to others and end up decaying society.
Basically, my response is this. While I wholeheartedly support the idea that religion is unnecessary for the rational human, who can see for themselves why one should not do wrong, it still seems necessary to have religion because not all humans are rational. Some people need religion to tell them how to behave, and so for them we must keep it.
I will end with a final note. Throughout this posting I have made reference to irrational humans. This is not to say that an irrational person is a bad person, in any sense of the word. Some people are born strong, some people are born intelligent, and some people are born irrational. A strong person may or may not also be dumb, if you believe stereotypes, but if they are, then it is a good thing for the intelligent person to help them out. The same may be true for the intelligent person, who may, if stereotypically weak, need help lifting things. The irrational person is like this too, in that they need the help of religion to tell them why they should do good and not do evil.