The best thing about this "assign after payment in full" clause: It makes potential disputes about unpaid invoices a copyright infringement issue. And crucially, legal costs for copyright cases in the US are paid by the infringing party.
(IANA lawyer, but I learned this from one while contracting.)
The most important part is that it avoids many cases where the client tries to "ship off" the software. (e.g. by selling it to another party for a penny and going bankrupt itself)
This is in some cases fraudulent in itself, but being able to block the sale is a much clearer case.
(IANA lawyer, but I learned this from one while contracting.)