I didn’t see it as a brag. The idea is that the cover letter influences the hire in undesirable ways more than it provides relevant information. Think systemically removed from the hiring packet rather than gleefully deleted.
We do use the hiring letter, because the ability to write is important to us, but I understand why it might not be considered.
This is very much counter to my experience as a hiring manager. The only time a cover letter has contributed to a negative response, the candidate stated they didn't care about the company and just wanted the money. That was also pretty much what they said in the phone screen, so yeah, not impressed. Other than that, we made some really good hires because the cover letter gave valuable context to an otherwise bland CV.
I understand. As I said, we value the cover letter and use it. In this case, though, the reasoning is that the cover letter provides unwanted signals about the demographics of the candidate, and avoiding influence from those signals is considered worth not knowing the kind of valuable context you mentioned.
It does not seem to me valuable indicator for technical position. Maybe if you are looking to fill position in marketing.
Even if you want to test communication skills, cover letter is completely unlike normal on the job communication for most of us. Cover letter shows how great you are at selling yourself. Technical person communication skills are more about clarity, ability to express complicated ideas, ability to simplify complex things for management, negotiation about deadlines etc.
We do use the hiring letter, because the ability to write is important to us, but I understand why it might not be considered.