I agree. The "exchange of work" tends to be fairer and easier than with just money (which tends to be in limited supply, you know).
I saw the thread in... not sure which but it was mentioned that if you paid someone to look for problems in your game there was a high chance of, well, getting problems regardless they were real or not. It's not something isolated: if you ask for your system's flaws, you will get flaws no matter how polished and tested it is.