According to some of my earlier tests done in version 6.06 or thereabouts: Depleted units do not have ZOCs, are not allowed to enter enemy territory/ZOCs, and are pretty much destroyed when an enemy unit moves adjacent. Broken units do not have ZOCs, are not allowed to enter enemy territory/ZOCs, may not use special movement (naval/air/rail), and are routed when an enemy unit moves adjacent. *Unready units do have ZOCs, are not allowed to move into enemy territory/ZOCs, and are routed when an enemy unit moves adjacent.
Note that each time an enemy unit moves adjacent to one of the above unit types, it has a victory check done. It is quite possible to rack up a few victories each turn chasing down broken/HQ/AirBase units. Note also that none of the above required any additional expenditure of MPs other than the ZOC for the *Unready units. So all that a blanket of empty shell units do is cost the Soviets the AP cost of creating the unit and freebie morale boosts for the Axis. Not a winning tactic for the Soviets in my humble opinion.
On the other hand, air recon will report them as units (rarely past a detection level of 1 or 2) so they can be best used in the Soviet Rear to seem as if a major buildup is happening in the area. Just hope that the Axis does not do a Panzer thrust into the group and watch them all disappear! This is one of Pelton's points above about the FOW effects.