The primary advantage of a solid certification is that it tells an employer:
- you have a minimum level of knowledge
- you have a minimum level of experience
- you've demonstrated successfully that you possess these qualities
- you've demonstrated a personal commitment to self-improvement and improvement of an area or profession
- the certifiying organization attests to that
There are all positive qualities and in the hiring evaluation process, I would assign a higher value to candidates with the certification than without the certification. If all other things were equal (e.g. organizational fit, experience, education, etc.) I wouldn't hesitate in hiring the certified candidate.
In general, I have found that the PMP certification is very balanced in its approach and run in a fair and consistent manner. People do attempt to "game" the system, but its usually very obvious during the interview process that they did not have an appropriate level of knowledge or experience when they took the test. Consider the candidate who didn't even know what PMP stood for!
So I think the only issue is with hiring managers. If you hire someone just because they have a PMP or other certification, you are not doing right by your company or yourself.