Comment 0 for bug 1019131

Quite simply because otherwise performance in the default unity/compiz session is awful. There is very visible tearing when moving windows, switching workspaces ect, ect.
Can't see why you'd leave it up to the users to enable thru xorg.conf

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: libgl1-mesa-dri 8.0.3-0ubuntu1
ProcVersionSignature: Ubuntu 3.5.0-2.2-generic 3.5.0-rc4
Uname: Linux 3.5.0-2-generic i686
ApportVersion: 2.2.5-0ubuntu1
Architecture: i386
Date: Fri Jun 29 00:27:49 2012
DistUpgraded: Fresh install
DistroCodename: quantal
DistroVariant: ubuntu
GraphicsCard:
 NVIDIA Corporation G86 [GeForce 8400M GS] [10de:0427] (rev a1) (prog-if 00 [VGA controller])
   Subsystem: Dell Device [1028:0209]
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Alpha i386 (20120627)
MachineType: Dell Inc. XPS M1330
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.5.0-2-generic root=UUID=4f62ae83-2284-48aa-8935-e1b260f3afcc ro quiet splash vt.handoff=7
SourcePackage: mesa
UpgradeStatus: No upgrade log present (probably fresh install)
XorgConf:
 Section "Device"
  Identifier "My Graphics"
  Option "GLXVBlank" "on"
 EndSection
dmi.bios.date: 12/26/2008
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A15
dmi.board.vendor: Dell Inc.
dmi.chassis.type: 8
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA15:bd12/26/2008:svnDellInc.:pnXPSM1330:pvr:rvnDellInc.:rn:rvr:cvnDellInc.:ct8:cvr:
dmi.product.name: XPS M1330
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.8+bzr3249-0ubuntu1
version.libdrm2: libdrm2 2.4.33-3
version.libgl1-mesa-dri: libgl1-mesa-dri 8.0.3-0ubuntu1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 8.0.3-0ubuntu1
version.xserver-xorg-core: xserver-xorg-core 2:1.12.1.902-1ubuntu1
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.7.0-0ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:6.14.99~really6.14.4-0ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.19.0-1ubuntu2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.1-1build1