Can I use Photon as an general purpose MCU w/o IOT functionality ?
Meaning we can still use some program on it without it connecting to internet ?
Toolchain instructions here:
Disable 
connection here:
http://docs.particle.io/photon/firmware/#system-system-modes
Yep, it is totally possible to use the Photon offline – just not by default.
Check out the Photon documentation regarding Systems Modes. By default is in the automatic mode which blocks the user code from executing without a WiFi/Cloud connection
SYSTEM_MODE(AUTOMATIC);
But you can switch it to semi-automatic or manual modes to allow your application to run without needing to be connected to the cloud at all. I use these modes even for applications that will connect to the Cloud so that I can get a better control over how my application functions when the network is unavailable for some reason.
SYSTEM_MODE(SEMI_AUTOMATIC); // before the setup() method
I find uploading new applications to the Photon to be more convenient over WiFi than having to connect everything over USB so even with my standalone applications I put support in to connect to the cloud when available for this purpose. No need to bother with setting up a custom toolchain and hooking up USB when you can use the cloud to do all that for you…the final result doesn’t need to rely on WiFi.