"a good developer should be able to take on all three roles" I generally would agree with you but it is not that simple. A good BA or PM can do lot of dirty work that the programmers can be shielded from in order to focus on their core value: code/build the product. Some of those dirty tasks are: 1. dealing with difficult business users who keep changing their scope and requirements. Can you imagine if you had to develop the product while the scope keeps changing ? A good BA/PM would negotiate that for you and help stop scope creep. 2. Looking at the bigger picture. This includes ensuring all the connected project members are delivering, communication is in place, people are talking to each other and everyone is on the same page. No one enjoys doing this dirty work but it brings tremendous value to a project.

There are more like these but you get the idea. Yes there are BA/PMs that do not do their task well but incompetence is not retricted to a particular line of work, is it ?