We have already discussed How Rhodey fit INTO the War Machine armor, but there's another major issue with other people using the armor(s): use/control of it.
In the movies, Tony seems to be almost completely dependent on JARVIS to fully control the suit. Tony commands JARVIS to activate things or divert power or repair systems. Some commands are just to make Tony's multi-tasking easier - others seem like he can't do them himself.
So we are left with two related questions:
- Can the suits be fully utilized without JARVIS?
- If not, why would JARVIS not just shut down the suit remotely - keeping Rhodey or anyone else from stealing the suit?