I don't think that intercreditor agreements are always necessary. I definitely think that sometimes the creditors rights are implied, or spelled out in other contracts.
For example, if one bank has a secured loan with the borrower, they would obviously have the right to whatever the loan was secured with. If another bank gave that customer an unsecured loan, they would have to recover their money from other assets in the even of the customers bankruptcy.
I imagine it might get a bit more complicated if the creditors are dealing with a business rather than an individual though.