If someone is able to get the id_rsa then it is able with no effort (?) to get the id_rsa's password from memory, because it's stored in the memory in a cleartext format?
It is not trivial to get access to the full contents of a machine's RAM. But if someone does have access to the contents of your machine, then that machine has been fully and completely owned, including everything on it.
Q: with FDE, is it worth it to use pwd on id_rsa?
Storing private information (e.g. SSH keys) encrypted even on machines with FDE is justifiable. Any process running on that machine in any context (including otherwise untrusted code) is not hindered by full disk encryption, as the OS transparently decrypts the disk on-the-fly.
Instead, full-disk-encryption protects the disk from being read "at rest", outside the context of your running system.
As to whether credentials are stored in-memory unencrytped, that'd be an implementation detail specific to a given product. But since there's really no other way to use the key except to decrypt it, then most likely yes.