If it’s for caching, can they not see which packages are most frequently accessed over http and implement caching that way? Or is that against their privacy policy? I’m not very knowledgeable on this subject but would love to know more!
As a software writer, it motivates me to know that my software is used and that putting effort in maintaining it will help people. For instance, I made a small Firefox extension that requires regular updates by design. Seeing that it was installed on a hundred browsers has motivated me to maintain it. If it was just for me, I probably wouldn't have bothered so much even though this extension scratched a personal itch.
As a user, I am a bit reluctant to being tracked so a balance must be found.
Edit: the amount of downloads from the archives is not enough. Some packages are downloaded again and again by automated systems like continuous integration systems and a downloaded package can be uninstalled immediately and this would be imperfectly detected.
Also using differential privacy you can gather statistics on package use while limiting knowledge about a specific user.