For one, without Google Play Services you have no Play Store. Unless you're going to prevent users from installing apps entirely, there isn't really another safe way to obtain apps. Additionally Verify Apps, SafetyNet, Safe Browsing, etc. are all part of Google Play Services. You _really_ want Verify Apps.
F-Droid and Racoon are ways to obtain apps. MicroG is an alternative to Google Play Services. How do these solve the other issues the commenter mentioned? Does MicroG include "Verify Apps, SafetyNet, Safe Browsing, etc."?
I believe the point is that such a signature is useless since the software signed as safe is actually unsafe, while a self-signed rom at least has a chance to be safe.
this reminds me of when team-teso had their stuff on their website directly accessible over https.. so they used a self-signed cert, so that no govt or corporation could require a MITM with a valid signed cert from any trusted CA.