Googlers are fine with admitting the small errors. I had a bug, this design isn't so great, the evals show this change to be a big loss. Those are the easy ones to admit. The big errors -- this product sucks, this idea is bad, eighty percent of the support personell on this piece of infrastructure are uncooperative jerks -- come less easily.
Anyway, eng doesn't really run Google any more. PM does. Eng can still push back to some extent and feels important when they control huge budgets for new datacenters. But who designs, specifies and ultimately controls the products for which those datacenters are being allocated? Most everything you see, good or bad, that happens on Google these days came from a PM.
What justifies a product management role separate from the developers? A PM that is also a developer understands better what is the cost of all these features.
Maybe a separate PM would see the forest better as he is not looking at specific trees, but there are a lot of people who can do both well.
Don't know about Google. I'm working for a defense contractor, so we may have more hoops and red tape to blunder through... but my PMs are always busy talking to the customer, writing documentation, figuring out what the specs actually mean, managing, or something. There's no time left in the day to hunker down with the code (or digital logic or whatever engineering discipline the PM came from).
Perhaps it depends on the industry, but in health IT I've yet to meet anyone who could perform both roles well. Developers mostly only make good PMs when the product is intended for either general consumers, or other developers.
Funny you ask, going through the middle of an organization where ProjM is turned into ProdM right now. As far as I can tell very little changes except the titles on business cards ;-)
The distinction I usually make is that a "project manager" is responsible for scheduling: maintaining a picture of the current state of the project, the current best estimates for when things will happen next, and make sure that everything that must happen before the next release is accounted for somewhere. It's not a "management" role in the traditional sense (people management), but it's not an engineering role, either.
A product manager's job is to understand why people are (or are not) buying your product. They need to understand the market, keep track of competing products, and be able to make decisions (or offer advice) about which of a list of potential new features are most likely to make the customer's mouth water. It's fundamentally a marketing role, but, especially with a complicated product, can require a fair amount of technical skill to do.
Disclaimer: This is all Big Company language, but not necessarily identical to Google's language.
Product Manager sounds better. It makes it sound more like you're responsible for shipping something that generates revenue, rather than overseeing an org-chart rework or some other corporate circlejerk.
Anyway, eng doesn't really run Google any more. PM does. Eng can still push back to some extent and feels important when they control huge budgets for new datacenters. But who designs, specifies and ultimately controls the products for which those datacenters are being allocated? Most everything you see, good or bad, that happens on Google these days came from a PM.