> You can set up a bare repository with no working directory
A bare repository has no working tree per se.
> I wonder if that is a requirement. I always thought that if you clone a repository you can just start using it as a remote without additional hassle
You can use bare and non-bare repositories as remote, but you should use only bare repositories (unless you know what you are doing). Here's a good explanation: http://bare-vs-nonbare.gitrecipes.de/
Sorry, you lost me; what's not right?
> You can set up a bare repository with no working directory
A bare repository has no working tree per se.
> I wonder if that is a requirement. I always thought that if you clone a repository you can just start using it as a remote without additional hassle
You can use bare and non-bare repositories as remote, but you should use only bare repositories (unless you know what you are doing). Here's a good explanation: http://bare-vs-nonbare.gitrecipes.de/