I have to agree with Gregg. Here is how I look at it..
Using your own example, (For an accountant, the project might be doing a client’s taxes) in a different perspective, ..just because I can do my own taxes, does not make me an accountant. Accounting is a process that we all involve ourselves in our day-to-day life.whether we do our weekly budget, taxes and so on.but an Accountant is a person who MASTERED the process. We all could be involved in project management on a day-to-day basis, but we may not have a clue about how manage a tricky situation in a project. A project manager who specifically gets trained to do that is the professional there.
As for, the concepts being same as general management; The general foundation of the medical profession is the same, but people ‘specialize’ in various areas.. just because a cardiologist and a gynaecologist both check blood pressure does not make either one of them ‘lesser’ ‘professions’.
However, I do agree with you that the certification does not necessarily mean that a person is the expert, especially with PMP. The belt definitely needs to be tightened there.