There's just no wiggle room to use unrecorded spousal waivers. We have to either have the spouse sign the mortgage to validate the lien or attach a recorded spousal interest subordination to the instrument. The only alternative is to use an unrecorded spousal waiver with an exception in the loan policy for the spousal interest and I just can't think of a lender that wants an exception for spousal interest, can you?
Underwriters are just paying too many claims on these cases that go to foreclosure. The unrecorded documents are too easily lost when lenders go out of business or do a poor job of maintaining their files. That along with title agents going out of business so nobody has a record that the spouse waived their marital rights.
The spouse may conveniently forget signing the waiver and there goes the validity of the mortgage lien. Poof!