According to the Wikipedia article on the Sokol spacesuit,
The Sokol space suit (Russian: Cокол, Falcon) is a type of Russian space suit, worn by all who fly on the Soyuz spacecraft. It was introduced in 1973 and is still used as of 2013. The Sokol is described by its makers as a rescue suit, and it is not capable of being used outside the spacecraft in a spacewalk or extra-vehicular activity. Instead, its purpose is to keep the wearer alive in the event of an accidental depressurisation of the spacecraft.
The KV-2 variant of the Sokol space suit.
What about the suit makes it not usable during EVA? Would wearing the suit during EVA mean instant death, or would it just mean increased risk?