2. In very small numbers (at most one in orbit at a time).
3. Recoverability of payload.
4. Very high delta-V (i.e. can shift orbits a lot, and has been observed doing so in big jumps during deployments).
The most likely guess is a testbed for military satellite subsystems; put a camera or a radar on there, and you can get a feel for its performance at different altitudes/angles, and also bring the prototypes back down to do a teardown after the fact.
