> Allwinner chips are lockdown-free even without this - you just need to hold down the right button/pin at poweron and the ROM bootloader goes into a mode that lets you load unsigned code over USB.
Side-loading unsigned 'ROM's/packages and gaining root are orthogonal concepts in Android: sometimes you want to obtain app-level root in the factory-installed OS - sideloading won't help with that (unless you are side-loading a rooted ROM, but that is not a given)
The boot ROM's recovery mode lets you load arbitrary code into RAM and then boot it. The conventional way to use this is load a flasher program that you then use to flash a new OS image, but you can use it to run any Linux kernel and and software of your choice purely from RAM, which can then do what it likes to the existing OS install. See http://linux-sunxi.org/FEL/USBBoot
> you can use it to run any Linux kernel and and software of your choice purely from RAM
As I said: sometimes you just want to run 1 app as root, not replace the kernel. Also, good luck getting a usable device with your own kernel/software when chip manufacturers don't provide drivers: unless you don't mind the usual litany of XDA caveats ("N0irROM v.0.4. Not working: Camera, GPS, Hardware acceleration. new: mic now working, randomly reboots less often")
Side-loading unsigned 'ROM's/packages and gaining root are orthogonal concepts in Android: sometimes you want to obtain app-level root in the factory-installed OS - sideloading won't help with that (unless you are side-loading a rooted ROM, but that is not a given)