I really don’t have anything against managers having reports write their reviews. I’m just pointing out there’s a fundamental difference between that and a referral letter, specifically on the “doing your job for you” bit people were talking about.

Saying the manager is also doing you a favor is twisting the words a bit. Sure, you can make the point that it’s nice and it benefits you. But it is a routine part of their current job. Lots of cover letters, maybe most, are being asked outside of this context. Maybe the most obvious difference is it’s within your discretion to decide to not give someone a referral. Not really the same with a performance review.

These differences makes an “okay, but write it yourself” situation fundamentally different.