It makes no sense to claim that work X is an arrangement of Y for a specific period of time - it doesn't really stop being an arrangement, and the only use for a start date would be the date it was arranged, which just duplicates the "arranger" relationship. Similarly for all other "other version" work-work relationships.

The medley relationship specifies to add an "arranger" relationship too anyway, so it's basically the same case as the others (edit: turns out dates are already disabled for this one).

For "parts", while it is sometimes the case that parts of a work are retired and new parts written, those changes generally mean a new main work should be added anyway, so having a date there is not the ideal solution either.