Because my counterparty may not be able to spend my IOU elsewhere.

This situation feels pretty contrived, do you have a concrete example, especially one which you've seen personally or been personally involved with?

not personally. but imagine you are the king of island A, and want to engage in a transaction with king of island B. Or you want to split up your estate amongst your children.

I get that, but I firmly believe that solutions, certainly ones as highly touted here, should operated in the problem domain. We've yet to identify, in 11 years, a concrete problem that blockchains are better at solving than any other solution.