For all the previously stated reasons plus game balance rage does not end at -1.
If it did, then at medium-to-high levels raging becomes a suicide tactic. You rage - you fight extra long, you go down to -1. Rage ends. You are now at, say, -11, from the loss of the temp con points. Whoops - dead!
Even without it ending, it's a tricky tactic - you need to know not only when to start raging, but when to end. It's all in the timing!