I have started using open dated tickets as my RTW usually spans most of the 12 month validity and I found when dummy-dating all segments I had to remember or remind myself to change the dates before the dated segment. With open dated you do not have to do anything until you decide you are ready to schedule the next segment when you call and make the reservation. I am not sure what would happen if you were a no show for a dated segment - but never wanted to chance it.

The other thing to consider is that when dummy dating all segments (after first international) you create one PNR which you keep for the entire itinerary. With open dated segments, you create a new PNR every time you make a reservation for an open dated segment on your ticket. This may get confusing and hard to manage - I know recently stupid me had made a reservation for remaining open dated segments on my DONE4 which had an extra segment not in my ticket - luckily I got away with a free re-issue for the additional segment.

Lastly, there had been reports of some airlines at some airports (eg BA) requiring re-validation of a dated segment with a sticker (even though date changes are free and revlidation are not needed) and charging for them - although such reports seem to have subsided.