I agree, this is very useful. Other than the advantages already noted, this is very easy to configure and you don't risk (potentially) misconfiguring your sshd in the process.
One thing this could be useful for is using a public server as a proxy to connect from a box behind a NAT to a box behind a different NAT. So you run spiped on the server and manually connect from client 1 to the server. Then you can SSH or VNC into client 1 from client 2 via the server. I wouldn't like to set this up using OpenSSH, but with spiped this seems easy.
One thing this could be useful for is using a public server as a proxy to connect from a box behind a NAT to a box behind a different NAT. So you run spiped on the server and manually connect from client 1 to the server. Then you can SSH or VNC into client 1 from client 2 via the server. I wouldn't like to set this up using OpenSSH, but with spiped this seems easy.