I don't put much stock in it because of the reasons you've mentioned Jerhart, however, it is a good thing for a lot of people who are not so well versed in forums or reptiles.
It's a nice thing for the newbs, gives some of them something to shoot for. Of course it will be abused but it's not like it has a real meaning, it's just a guide, a who might be worth listening to of having advice of any value. Once you've been around awhile, as you have, you begin to see the drawbacks. You can have your pals hit you, trade with them, and bump up your greenies.
I get pms from people now and then asking if I'll give them rep if they give it to me. I explain that I don't play that game and move along. I would imagine there are some who do give the same answer.
It really has no significant meaning but it is one of those nice things that makes people feel like there is something they get for what they give. There is a LOT of giving here. That does rate a little something which the rep system is; a little something to make you feel good and give a bit of an indication of what, when combined with how long a person has been a member, they may know.
Don't look for fair, it isn't there. It is a nice thing though.