I don't want to turn this into an engineer vs. PM thread, but since YOU asked (

-- and keep in mind that I am still considering switching to a PM role at some point, so don't take it personally).....
In my case, I have first hand experience with various PMs with PMPs and they were generally worthless. They added nothing to any of the projects and generally resulted in delays, obstruction, etc. It would've been far more efficient to have a lead engineer run and plan the project, which is what I've done in the past and had far better project results. I've been on projects run by PMPs and been on projects run by engineers (including myself), and the projects run by engineers were always the ones that were managed better.
For example, a PMP I am dealing with now is driving me crazy and I finally got fed up with him last week and told him to butt out and I would update him when updates need to be made. He is with an external company that did work for us and now, the product is in our hands for a pilot and it is completely unnecessary for him to badger me via email, phone, and visits to my office especially when those calls/emails/office visits inevitably result in me having to re-explain concepts or plans for the tenth time. Seriously, when I talk to him, I feel like I am talking to a 6 year-old and have to talk S L O W L Y so he'll understand. Again, an engineer managing the project would be a better fit.
Another example is that we hired a PMP at our company as a contractor and I fear he will become full time. The guy is clueless and when he first started, every discussion with him turned into an argument about what Sharepoint was capable of. He lost EVERY SINGLE ARGUMENT and yet, kept arguing with me and arguing with me and is another that you have to explain things to multiple times and it gets very, very tiring.