In the comments to yesterday's story, Timo Aaltonen of Canonical and part of their X/graphics team responded. "no fglrx, can't force people to switch to radeon and likely regress, on newer hw at least."
So Canonical is keeping away from using the latest X.Org code since the Catalyst (fglrx) driver doesn't yet support it and they don't want to regress users by forcing them to use the improving but still less than perfect open-source driver. Canonical's effectively bowing down to a binary blob.
Historically the Catalyst driver doesn't support new Linux kernel or X.Org Server versions until right ahead of the next Ubuntu release (or release of other major tier-one Linux distributions). In the past there's been several instances of AMD having to pre-release Canonical a beta driver to ship in the Ubuntu archive for compatibility with an imminent release with not having a published AMD.com driver that supports the latest code and in the interim force users to the open-source AMD driver, but this time the xorg-server-1.16 landing is being held up instead until there is this proprietary driver support. NVIDIA meanwhile has already supported the X.Org Server 1.16 ABI for quite some time along with all of the latest kernel releases.
Forcing users to use the open-source AMD driver temporarily until X.Org Server 1.16 support is in place would mostly mean slightly lower performance and being bound to OpenGL 3.3 support rather than OpenGL 4.3/4.4 (though that doesn't mean much for most users) and perhaps some bugs, but it shouldn't be too serious with one exception. The main people that the lack of Catalyst support would hurt is those with the high-end Radeon R9 290 series "Hawaii" graphics cards where you need to be using Mesa 10.3 and Linux 3.17 for proper support... Canonical decided to go with the Linux 3.16 kernel and won't officially be supporting the Linux 3.17 kernel on Ubuntu 14.10, so unless they end up manually back-porting the Radeon DRM code, there won't be official AMD R9 290 Hawaii support in this next Ubuntu release. Open-source graphics driver users would also be better off if Canonical would more quickly update against the Mesa 10.3 code rather than sticking to Mesa 10.2, etc.
On the opposite side, switching over to X.Org Server 1.16 would help open-source Linux driver users. X.Org Server 1.16 has the built-in GLAMOR support that's much more refined over the older external library and it's expected to be much faster. GLAMOR is required by the Radeon HD 7000 series users and newer so once switching over to the 1.16 support the 2D experience should be much better. GLAMOR is also needed for the GeForce "Maxwell" support within Nouveau and requires the X.Org Server's built-in implementation. X.Org Server 1.16 also has many other new features including built-in XWayland support (though not too relevant to Ubuntu users now in light of Mir), improved support in the ARM space for non-PCI devices, many clean-ups, etc. But for now this open-source software on Ubuntu is taken hostage by AMD's binary blob, which we can hope will support X.Org Server 1.16 quite quickly to avoid having xorg-server 1.16 potentially miss the Utopic Unicorn cycle or arrive late and lead to a bug-ridden experience.