Describe what you mean by "force the use of serial numbers". I would interpret this to mean "force the user to enter a serial number when shipping an item in OE (shipping occurring when you invoice the order)" and my answer would be the one Andrew gave you.
I'll throw my 2 cents in. I do not use this feature because when we move items to shipped, we can create the serial number line but must leave it blank and print a picking slip to the warehouse (we do not know the serial number yet). The warehouse staff picks the item and invoices the order, weather they enter a serial number of not. My defination of forced is that the order can not be invoiced until the serial number is entered.
The picking slip tells the warehouse how many on an order to ship. I don't want the warehouse staff deciding which customers are getting the products and which ones arn't. In our system here, the warehouse staff picks the order, invoices it and sends the invoice with the product to the customer.