[cdjk]

Even if there aren't people who have "Product Manager" on their business cards there's going to be someone acting as a product manager.

In my possibly overly-simplistic view, PMs should generally focus on what the customer should be able to do, while developers should focus on how it's going to work. Of course, each group needs to be able to understand the position of the other to be useful. I'm worthless as a developer if I have no empathy towards for the customer (i.e. I shouldn't say "we're not going to implement this feature that would be useful to customers because it's too hard"), and PMs are useless if they don't understand the technical constraints of the existing code/infrastructure.

I'd say that PMs are less technical than developers, so developers can do the PM work if necessary (and should be thinking about it a little bit even if there are good PMs).

Project/Program Managers are different. Their job is more focused on making sure all the moving parts of a "project" are coordinated. When it's a small start up there aren't as many moving parts, so it's not a useful role - but in bigger companies it's useful to have someone coordinate between marketing, legal, finance, customer service, etc. And they can make sure everything happens on time.

You run into problems when developers don't care at all about customers, product managers have no sense of what's technically possible or reasonable, and project managers get too hung up on the process and not the final result. Avoid those situations and everyone adds value.