1) Nonshell use only -- you want port forwarding turned off. Unless you're using the machine as a proxy, it's just waiting to be used as part of a larger hack scheme.
2) Shell use only -- Normal logging in and shell use doesn't necessitate port forwarding. The only time it is generally useful is for forwarding X11 back to the client, but frankly that's not nearly as useful as it was 10 years ago. If you've got an X install on your server, and an X server on your client, then you're in a sufficiently-select subset of the user population to have to turn on one config option in sshd_config.
In either case, I think it should be turned off by default.
