Some people denounce all use of agent forwarding because, by default, ssh-agent doesn't confirm with the user before signing a request with the users private key. This means, if you ssh into a compromised/malicious host with your agent forwarded, malicious code on that machine can just silently ssh into other servers as you.

This trick has been used in the past by blackhats to escalate from a compromised CI environment to full production takeover.

GauntletWizard probably meant to respond to your parent in this thread.