Is it more likely that you want to have the exact same need date, or that you want to set a new need date on the new shop order relative the original shop order?

Would be interesting to hear the opinion of above from you and also other to see the most desired behavior when duplicating a shop order. 

Not sure if this will help, in Apps9 I find the result is different depending on the status of the SO being duplicated. 

Open Shop Order populates the Need Date with a duplicate. 

Closed Shop Order populates with a calculation 

Just questioning why the functionality changed. Maybe it is seen as an improvement by some customers, others maybe not.

In my opinion, duplicating should create an exact copy of the source data. Duplicate a customer order header (not copy order) or a purchase order header, everything is the same. Duplicate a PO line, everything is the same.

I would say it is a good thing as it prevents you from saving a wrong Need Date.