Yes, the ability to adjust schedules is valuable. However, it is an ability which is reduced as one's shop-loads increases and when one completes shops across larger geographies.
I might not, for example, feel like completing the shops I have scheduled for a given day but by pushing them to later in the week might mean the need to shop without breaks from the early morning until after dinner or the need to lump the 75 miles drive to the east of town which was planned for one day and the 75 mile drive to the west of town which had been planned for the next day. Worse still is having one's schedule so filled going east and west that one is unable to accept those rare emergency requests from schedulers - 'I had a flake yesterday, can you do a shop 75 miles south of town. It has to be done today. Would an extra $100.00 on top of what we paid you last time you went there be enough or do you need more?'
Additionally, many MSCs require firm shopping dates upon assignment. Many others require that rescheduling be coordinated with a scheduler, many of whom are not immediately available. Some even get bent out-of-shape by requests to reschedule an assignment in order to complete it earlier than originally planned.