I think the proxy is a bigger benefit than the checksum database. For the CI use case, it lets you get your modules from a server that's under less load than github (which has to support writes). Your go.sum already contains the checksums, so the sum database is not relevant there, but the module cache adds a big reliability and speed boost to your workflow.

The checksum database is of course extremely important (you want to get a known-good copy the first time you use a module), but I stand by my assertion that it's beneficial for both to be turned on by default. Yes, it's a small privacy leak. But not one that is particularly personally embarrassing or useful to adversaries. So I think the go team made the right call.