This part here is key:
The diplomat reiterated that nuclear deterrent was the only guarantor of Russia's sovereignty, and Moscow would never allow it to be put under threat.
This is a real perspective in Moscow. They think that if they have a major case of internal instability in the North-Caucus and decide to deal with it in a violent and decisive manner, the west could conceivably interfere (be it for human rights, politics, or both) and the only way to prevent it is by maintaining a nuclear deterrent. They also believe that nuclear weapons gives them a certain freedom of action, in that the west will be very careful to avoid direct military confrontation.
EDIT: Serdyukov just stated that all Russian suggestions on joint-BMD have been turned down by NATO, and at this point the MoD believes that current Euro-BMD plans could damage if not effectively neutralize the Russian nuclear deterrent by 2020. He says that in this case Russia will work to create means of defeating this BMD.
NATO at the same time says that they can not have NATO member states relying on Russia for their BMD, which means that Russia can participate, but certainly can't provide any vital components to the system. Rasmussen suggests that Russia and NATO develop two separate BMD systems, that would then cooperate, exchange data, etc.
http://newsru.com/world/08jun2011/serdukov.html