not at all. GNU screen requires changes to your server. Also you would need to change screen and run it as root unless you're really smart (in which case you wouldn't need support now would you?).
Screen requires you grant access to both users to the server (auth issues), that both users can connect to the server (firewall issues), and doesn't provide any protection or auditing for the "master user".
In short, if you want simple and secure server support, you absolutely do not want to use a hacked GNU Screen.
Your question is somewhat analogous to saying, "why would I want to use GoToMyPC, WebEx, or CrossLoop instead of just installing and configuring VNC for free?"
ShellShadow is the simple, secure-by-default way to do server support that does not require any changes to your server, does not require you to give out auth credentials to your server, and plays well with firewalls.
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.
The PuTTY code base is not structured for "add-ons". You can only take the source and modify and release something new.
I can say from my experience developing http://shellshadow.com that the PuTTY source is readable, lightwieght and well structured.
Many thanks to Simon Tatham for such a simple and long-lived tool!!!
