[m45t3r]

What you mean by "usable"? Vanilla Android is really usable nowadays (I have a Nexus 6P with Android 7.1.1, non-rooted with locked bootloader since I don't really root nowadays). CyanogenMod nowadays is more important for their support to multiple hardware then the customization from AOSP per see. This is especially true since for those who really want mods, things like Xposed Framework offers much more customization.

[gsnedders]

"Vanilla Android" (from the AOSP) doesn't pass the Compatibility Test Suite (CTS) nowadays, so according to Google's Android trademark rules shouldn't be called Android. Some of the "Core" apps simply don't work at all.

Don't confuse what Google publish at the AOSP with what they ship on the Nexus/Pixel devices: they're increasingly different, with AOSP increasingly dysfunctional. Heck, the initial release of Android 7 for the 5X/6P at the AOSP wouldn't even compile because it had closed source compile-time dependencies.

[m45t3r]

I know that AOSP isn't the same Android running in Nexus. However I have an Android tablet that I used to run AOSP (nowadays is running CyanogenMod, probably needs to change it to LineageOS). The core experience is exactly the same once you install GApps (at least the minimum necessary to run Play Store).

And no, I really don't need anything that CM brings (I only switched from AOSP to CM because it was better supported on my tablet, CM actually had a developer while the AOSP guy was simply pulling the changes from the CM developer).