As I read it, the complaint is that you can't revert to the old OS if you install a new OS.
Show me any OS installer that does that!
Well, as usual a bunch of people who have never flashed a rom to their phone and don't know fuck about shit are here to comment.
Let's talk about the Nexus 4 in particular. I own one and have been flashing it regularly (heh heh) for days now. This is not my first time around the block, but it is my first Nexus device. The Nexus 7 (also have) is basically the same, a little harder to get into the bootloader. If you flash cyanogenmod to the device via ROM Manager, which requires a compatible (TWRP or CWM) recovery to be flashed first (first you use adb to unlock the bootloader, then you use fastboot to flash a custom recovery) then it will offer to make a device backup before you install. The sdcard partition is maintained, so your backups persist. So do any application backups made with TitaniumBackup or similar. These backups can then be restored with the same recovery in which they were produced (last I looked, CWM and TWRP had slightly different backup formats) and they will restore the kernel, recovery, system, data, and even cache partitions.
In short, the normal, on-device installs of CyanogenMod do make a backup of the OS, and you can revert to the old OS. I can restore right back to 4.2.2 if I want; I also made a backup with the 4.3 factory image and with CM10.2, and have saved the backups aside (via adb) so that if I wanted, I could restore one of those backups and be off and running.
This fact makes the CyanogenMod installer's refusal to perform a backup during install not just inexplicable, but unacceptable. The functionality is already in the recovery that is flashed with CM! They don't even need to add a reboot! Just flash recovery, boot to recovery, make a backup, then install the CM ZIP! Done and done! That's what I do every time I install a ROM, unless I'm not that enamored of my current system. After all, I can always restore an older backup, restore any newer apps from TitaniumBackup, and then update any remaining packages from the Play Store — then back those packages up with TitaniumBackup.
Whinging from an idiot in my books.
And yet, your post is a tale told by an idiot.