New Linux users, even those who are competent within Windows, are just too intimidated by a CLI.
That said, the simplified config for Screen is pretty nice, especially for those of us who regard mouse use as a cache-miss.
I believe this is targeted at system administrators, and not the average desktop user.
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Edit: as a regular user of Screen, I cannot pass up an opportunity to promote it. It is most definitely one of the most amazing pieces of software I've ever used, CLI or not. The ability to run multiple terminals inside a persistent session is enormously useful.
I not only use it for software development (where split-screening the Screen session gives me the best development environment I've ever used) but also for running all kind of other applications. I use rTorrent (http://libtorrent.rakshasa.no/), Irssi (http://irssi.org/), ncmpc++ (http://unkart.ovh.org/ncmpcpp/) with MPD, and CenterIM (http://www.centerim.org/), all in a Screen session on my always-on desktop, which lets me turn off my laptop whenever I want without closing any of those applications.
I doubt that sysadmins will ever be considered "the masses".
That said, I've probably had at least one screen running somewhere in the last fifteen years.
I actually don't think I've used the splitting feature, I'll have to give it a whirl.
Make sure you get an SVN build, so that you've got vertical splitting ;)
To any Linux developer/admin who hasn't used it - if you want to push your terminal wizardry to the next level, make a concerted effort to learn Screen. Once you get the basic commands down (and these Ubuntu modifications should help the learning curve) it really does save a lot of time.