For me there is an issue in the division of responsibility. The payment term can not be responsible of the starting date of its computation because it does not have any link to other document. And more over invoices are not always related to any shipment.
So for me it will be good to have in standard a payment_term_date on invoice which override the default starting date. This will also solve a common issue for supplier invoice which is that companies do not always now how the term is computed and so this will allow to just fill the term date (without any payment term).
By default in standard the payment_term_date is empty but it is easily to customize the purchase or sale to fill it with whatever people want.
But nothing more should be added in standard.