One of the UDS Copenhagen sessions on Monday was about a testing environment for Ubuntu that doesn't need 3D support. "At various points in the development cycle, we don't have 3d acceleration everywhere we need it (in VMs, on ARM). We need there to be a solution for being able to test desktop applications continuously during these periods, even if we're not able to test Unity itself at the time."
These issues were covered in detail on Phoronix already via Not All Linux Users Want To Toke On LLVMpipe.
Rather than maintaining Unity 2D or some other desktop, some developers during the UDS session basically decided to just use Lubuntu, the LXDE version of Ubuntu. LXDE doesn't require a compositing window manager or any form of GL support and is very lightweight. One of the reasons for Lubuntu over KDE with Kubuntu or Xfce with Xubuntu is that there are ARM images of it already available.
Disabling of Compiz effects was deemed not doing enough to improve performance and would slow down Ubuntu testing, the GNOME Fallback code isn't maintained so that's not an option, so basically the only real choice was to use another supported flavor of Ubuntu. They also didn't want to "commit to maintain extra components or invest work in a fallback solution if not needed" nor do they want to ship a fall-back desktop on the Ubuntu CD.