Ok, yeah, you've got a point I think. But one could argue if microg is enabled by default, at least some info might leak to google as their push servers are contacted and a device id is created (even if the data is anonymized to some extend.). (Depending on if these settings are enabled by default in microg which I am not sure of).
Here's some info from the divestOS faq (cmp.: https://divestos.org/pages/faq):
"Anything important I should know about microG?¶
The 'Google device registration' and 'Google SafetyNet' options WILL make microG connect to Google servers.
The 'Cloud Messaging' option WILL make microG maintain a persistent connection to Google servers.
The 'Cloud Messaging' option does NOT require a Google account.
The 'Google SafetyNet' option WILL download and execute proprietary obfuscated code from Google and is strongly NOT recommended.
While microG itself is open source, any apps talking to it will do so using the proprietary Google Play Services library."
It goes on to provide some guidelines if you want to use microg:
How should I configure microG?¶
"Depending on the apps you want to use there are a few different ways you can use microG.
Some apps don't need microG but check that they were installed via Play, in this case you only need microG Companion/FakeStore and to install the app via `Aurora Store` (via session installer) or `Obtainium`. This mechanism only works on 18.1+ currently, adb workaround still necessary on older versions.
Some apps will work with microG simply installed without any Google connections, in this case it is strongly recommended to revoke Network permission from the microG app.
Some apps need push notifications via Google, for them you must let microG maintain a persistent identifiable connection to Google. Enable 'Google device registration' and 'Cloud Messaging' in microG.
Some apps require a captcha to be performed by the user, for them you can enable the 'Google SafetyNet' option.
Some apps require SafetyNet to work, while the option to enable it currently exists it will not work in the unprivileged mode that DivestOS uses and will be removed in a future update."
So depending on your thread model, you still would want to disable some of the options in microg to have absolutely no leakage of data to google. For example I am not comfortable any more with using push notifications since it was revealed that state actors use this info to tail users communications.
Could you explain why you wouldn't use it?
I've been using it for a couple of years and am happy with it, it grants an extra layer of security I think, if you can wipe the device when lost/stolen. Also very handy if you misplaced the phone and its set to not ring, as with this it will ring at full volume. You don't need to use their server for the app to function, if that is your concern. I use a secondary device from my household. You can send a text message to your phone to let it ring even when its set to silent mode/get its location/or even wipe it remotely.