Users of my (free, open-source) app seem surprised to learn that we've got zero insight into usage patterns. There are situations where a small amount of anonymous telemetry would be extremely helpful but I'm not going to touch it with a barge-pole. Opt-in makes the data useless - not just in terms of the huge drop in quantity but because of the fact it introduces a huge bias in the data selected - the people that wo…
Why? I don't think that's obvious. It may also be related to the way the opt-in is presented. In general, I would expect this to be a workable solution. Even if the opt-in group deviates from the "typical user", it's the best data you can get in an honest and ethically sound way. This should certainly be better than no data at all?
For any website/app that presents an opt-in cookie consent banner this is implicitly already the case.