Patches have just landed1 on inbound that remove most of the Add-on SDK. When these merge, it will no longer be possible to run SDK add-ons on Nightly, or use most of its modules from Firefox code.
The only parts of the SDK that will continue to ship are the module loader and a small number of utility modules that are still used by devtools. In the near future, most of those should be phased out too, as devtools phase out uses of the remaining modules, and move to a simplified loader without the overhead and baggage the SDK loader has maintained for the sake of add-on compatibility.
If you run into any issues as a result of this removal, please file bugs blocking 1.