I have worked with good and bad project managers and the best ones came up from being developers. You have to have some knowledge of what you are attempting to manage or you are going to be worthless as a project manager.
Too many companies jumped on the PM bandwagon and hired a lot of worthless baggage. Worse than that, they gave them complete power over how the solutions would be developed, an iron fist over the project schedule and tied success only to how well the schedule was adhered to regardless of the quality of the software. There is no flexibility and little input from the developers with the knowledge to do the job.
PMs are great for overall management and tracking of projects, going to the meetings and managing the paperwork -- things that would take developers time away from the actual project. The big problem is when a PM is made "the boss" of the project and the developers are forced to adhere to the "bosses" way regardless of whether it makes sense. Too many of the PMs I've seen couldn't develop software if their lives depended on it and have no idea of what it takes to develop good software. They are another version of the military's REMFs.