Dates on event relationships mostly duplicate the event data, which is lots of extra work for editors to add and update these dates, and lots more opportunities for mistakes to be made or data to become inconsistent.
For single day events, the dates are already always duplicated data. The only time it's useful to have dates on both the event and the relationship is for things which only occur on certain days of a multi-day event. Now that we have the "parts" relationship for events, we have a way we can enter single days of a multi-day event without needing to have dates on a relationship, so I think only having event dates would make more sense.
If we are going to do this, I think the decision should be made soon. The longer we leave it, the more data we'll have and the harder it will be to fix it all before we can disable the dates.