Screen is a great tool, I use it regularly myself. ShellShadow is not a replacement for screen and screen's "collaboration" features are secondary to the tool's original purpose.
There are problems with using screen for collaboration...
...how does the "other person" access the screen? You have to give them a id and password to the server, right? and if you are doing SSH auth properly, you shouldn't allow password access, you should be using keys. So you have to set up a public/private key pair for a temporary support technician if your server is secured properly, right? Ok, now they can login without you, right? How do you control this second user's interaction? How do you audit what the support tech did and differentiate that I/O from what you did?
ok, lets assume you actually want to do the above. Now, is this server accessible from where the support person is? If your server is inside a corporate firewall, it isn't, right? If you have SSHd locked down well, it shouldn't allows access from unknown IP addresses, so they won't be able to get into your shared "screen".
I could go on. The bottom line is "screen" works fine for sharing your terminal session if both users have login access to the server and the shared interaction is highly trusted, informal, and you have the time/skills to administer this solution.
ShellShadow works with zero mods to the server and you do not have to give login credentials to the support tech. With ShellShadow, we relay/collaborate from the terminal "client". The terminal "service" (SSH, telnet, etc) is unaware of the interaction.
