The main reason for it is that I feel a lot of screen real estate pressure, and the taskbar pressure. I used to have everything placed in such a way that there was always a little corner of every application into which to click and bring it forward. However, as application hackers became greedier and greedier, I had to resort to running some of them (e.g. Sylpheed and Liferea) in full-screen mode. Miraclously, I still do not have a maximized Firefox.
A secondary reason for using screen is how well it allows to group sessions. Typically I open one terminal window per remote host, which is natural.
I tried tabbed gnome-terminal at some point, but it wasn't a good experience. Since the terminal gets obscured, it's twice the work to select what you need: first pick a taskbar button, then the tab. And finally, screen provides a degree of protection against issues with the terminal and connection reliablity (VPN over vpnc teaches you to start screen before vi very quickly).
I suppose that using a virtual or paged desktop might be an alternative, but every time I tried it, it was too cumbersome. These attempts go back to the days of olvwm, and the last time was a month ago. This paradigm is just not for me.
