I definitely want tasks to be ordered too. I don't see as necessary the task numbering, but could be there if configurable.
About your questions: the 'order' field was used on the old tasks interface (around version 0.8 and older) when tasks could be ordered. I don't know why that was dropped.
To order tasks now you have to use either the start date as you suggested or due date or name. The problem with any of these is that to change the order you may have to change several tasks. E.g. If you use the start date to order 10 tasks and give them consecutive dates, then to make the 10th task become the 5th one you have to change the dates of tasks 5 to 10. You can take some precautions to avoid this, like leaving several days in the middle so that you can then move a task to any place., but this is just a workaround and not a real solution to the problem. The real solution would be to implement task ordering and let the application handle the details.