Ray_N - yes, I understand that point, it is unfortunate. I think the general principle would be that if (read the criteria closely again) the thread has spiralled off topic, and as the rule says "without any new posts on the original or related subjects", then there is no further useful discussion, its just a flame-war. So we lock. It's no immediate block on the usefulness of the rest of the thread, as there are no further useful comments at that time.
At some point in the future (and I agree, this could be the next day!) if someone has got some useful content, I am perfectly comfortable with unlocking the thread at their request.... Hopefully there will be so few threads locked due to flame-wars that this won't be a burden. I would suggest the work practice would be for the mods to unlock the thread and then eliminate the offending posts, so that the new post can carry on seemlessly without the intervening, diverting posts.
We can't codify everything, that needs to be a judgement call - if the thread topic will benefit from being reopened to new contributions, and the clarity of the thread is improved by removing those offending posts, then do it... it improves the quality of the forum.
Seer - I agree. This was a suggestion, so I put it to the vote. The more we put out there for agreement, the more respect and mandate the final charter will have.