I have hosted swaps on another site and I have participated in many swaps here on FAOL. As the swap host I stick to the due date and if flies are received after the due date they go back to the tier. There is no sense having a due date if I extend it waiting for late flies. Swap hosts give us ample time to tie and I do not join if I do not plan to have them in ahead of the due date. I consider the "flies must arrive no later than..." to be a business agreement. Once I had to ask out of a swap because I was so ill for a long while. I felt bad about that but there was nothing to be done about it. I expect there are other valid reasons, but for most of us most of the time the due date is there for a reason.