Using ssh-agent also means you can practically put a very large pass phrase on your SSH key, because you'll only type it infrequently. Good luck brute forcing my passphrase.
I'm pretty sure ssh-agent doesn't store the password, but the private key. Also, the fact that it supports timed expire (and can be setup to drop keys upon events such as screen lock) make it a wiser choice than passwordless keys.
That's correct. And ssh-agent doesn't give access to the private key either, only to perform operations like signing. The only way to extract the key is to search it in the process memory, which I believe would requires root level access.
2. In my experience on Debian (granted this was in 2010), there is a noticeable performance difference between `htop` and `top`.
4. ssh-agent stores the password in memory and is erased on reboot. OTOH If you use a passwordless key file, anyone can use it if they have the key.