Now, this does not mean that the project manager does not need to be involved with driving the project and the requirements gathering and even, in many cases when you are working on internal projects, be the person actually documenting requirements. But it is noteworthy that when the project manager acts in this capacity, they do it representing the customer organization They should be aware that they are wearing two hats and may have to develop a split personality. So while in real life this often happens, it is preferable to split the job of requirements gathering between the business analyst (or whatever you call that function in your business) for all product related requirements, and the project manager for all project related requirements. These requirements should be documented in two different documents with different ownership. The product requirements goes into the BRD (Business Requirements Document) and the project requirements belongs in the project plan.
It is also clear that the process of collecting requirements in the PMBOK 4th edition could use some enhancements. The inputs are the charter and the stakeholder register. While this may be defendable by making assumptions about what goes into the charter, I think it is noteworthy that, without a solid understanding of the existing business architecture, goals and objectives, strengths and weaknesses and much more, the collection of requirements may focus too much on the project and not enough on the business. And after all, a project that does not help the business achieve its goals is not a very good project.
So... after all the complaining, can say nothing positive? It is clearly good to focus attention on requirements. Many projects fail because of poorly defined, continually changing, and misunderstood requirements. It is great that PMI recognizes and highlights this. And there is no doubt that the project manager must be a major player in this area. Just remember that the project manager is not necessarily the best person to capture and analyze what the customer wants. After all the expertise a good project manager brings to the table is how to implement those requirements.
Good luck and challenge everything!