Useful does not equal ethical or legal. A lot of things would be useful to me.
Here's what I think would be a closer analogy to what's actually being proposed: the water company installs a smart meter allowing them to read usage data more frequently. They randomly sample 2% of customers' data and prepare a histogram showing the top daily usage hours and total usage over a monthly period.
Probably the data is too wild to make any use of, but then all the folks who have a clue using the playground as a kind of proof-of-concept play area... would be great for analysis.
Anyways, I'm not sure how well it would work, regardless of if it should be a thing at all. The majority of stuff I do, and the folks at work, we all have build pipelines that control internet access, use cache systems, etc... so I'm not sure it even matters.
The main thing I think this would do is help them flesh out the non-mainstream configurations. For example, I've used Go on AIX but I'd be surprised if bug reports on that platform didn't hit basically every maintainer by surprise since statistically nobody tests on it regularly or even has access to do so. I'd think most of the value of systems like this would be finding out that the change they tested exhaustively on everything Google normally uses is also going to break 100% of some niche community.
Where I'd expect difference is precisely over the question of whether it's worth the cost & trust issues. That makes me wonder whether there might be some middle-ground here where some third-party like the Linux Foundation could run a telemetry service which is highly public in both its design and collected data, and that would be enough that many people would be comfortable with this kind of service without Google's business reputation entering into the equation.