Is your main objection to it that it can obfuscate login activity since many systems track login/connection events at the sshd level and are oblivious to SSH multiplexing?

No, it means anyone that can get your team to execute a script can log in as you in any data-center you have authenticated to regardless of multi-factor authentication without using credentials. It means firewalls do not exist, CVE's not required and credentials are not required.

I personally find it extremely useful

Absolutely, not using credentials and riding the existing channels will always be faster. Removing authentication requirements will always reduce friction.