It makes sense; not everyone has long legs for running or is big or strong enough to fight. I think the tonic immobility is more a "I'm dead, maybe you won't notice me" reaction or the nice dreams/lack of fear when we're freezing, drowning, have the hungry predator breathing down our neck already and the other two don't make sense. It's a deer in the headlights response and I know I've reacted that way; can't think or "do" anything useful. I was watching a TV Discovery science show about the human body the other day and it was talking about how the human mind has a last trick during emergencies where it literally slows time so your brain can "decide" on THE best option to keep you alive, takes over so there's no conscious control.

http://dsc.discovery.com/tv/human-body/human-body.html

(Episode 4, "Brain Power").