I think there are different meanings for the phrase “bug advocacy.”

I agree with your feeling that some testers try to usurp the role of the product owner, and that testers who do this add negative value to most projects. http://kaner.com/pdfs/I%20Speak%20for%20the%20User.pdf

On the other hand, I see too many testers who don’t accept accountability for the quality of their own bug reports. I see a bug report as a persuasive document, one that provides the technical detail and (if available) market-relevant information that is useful for the other members of the project team to make an informed evaluation of the importance of the bug — and to have a head start on troubleshooting it. The process of creating that report is what I see as bug advocacy. http://testingeducation.org/BBST/bugadvocacy/BugAdvocacy2008.pdf