Opting out of background refreshes would mean that fetching a module version that (1) no one else had fetched in a few days and (2) does not use a recognized open-source license might not be in the cache, which would make 'go get' take a little extra time while the proxy fetched it on demand. The amount of time would depend on the size of the repo, of course.

The background refresh is meant to prefetch for that situation, to avoid putting that time on an actual user request. It's not perfect but it's far less disruptive than having to set GOPRIVATE.