Darren, you were right the first time, I just didn't understand the problem. I do now, today, and only because I got a phone call from AA, about the very same situation you described before and I failed to see.

Two days ago I made a reservation, with the first three segments dated, the rest totally open. Today AA rings me and starts blabbing about my tenth segment being "invalid". I said "what on earth do you mean invalid, how can HKG-AKL be ėnvalid?". The agent says "because you can only cross the Pacific once" ... ... after I picked myself off the floor I managed a timid "pardon? HKG-AKL is a transpacific?" She says "YES, (with a tone usually reserved for a five year old with a learning impediment), of course, because you are connecting at LAX (HKG-LAX-AKL)". I am?? ... anyhow, to cut it short, it turns out that all 17 of my 'open' segments had been actually dated, by AA, with flights and dates pulled out of thin air by someone at AA with a vivid imagination. Needless to say many of those gems of dummy flights were way out, and naturally caused the rate desk to have a coronary.

So yes, you are right, they do have to date all segments, for their own internal validation purposes. The ticket stays open, but there are phantom dates/flights behind those open sectors.