The agent requirement is a symptom of a deeper potential issue: not using the system's ssh. Providing built-in as a fast-path option is fine, but ~/.ssh/config can do a lot of useful things, and providing an easy option to say "just use real ssh" lets people have their config just work.

Specifying identify files for specific hosts, using bastion hosts (ProxyJump), etc. For example, my laptop SSH config detects when I'm not on my university network or VPN and adds a ProxyJump through the department bastion host, but connects directly when I'm on the network. Configurations like that break when software makes too many assumptions about how ssh is configured.