This makes sense to me, especially in the case of a foreclosure.  The lender cannot foreclose on more property than that which is encumbered, and would not want to enforce the lien on less than that which is encumbered.  They are also asking you to compare the legals and to note any differences, which would give the attorney an opportunity to seek a remedy for any error.

Here's an example of a case where using the legal from the vesting deed might cause problems:  Eminent Domain.  Say, in 1990 the borrower purchases Lot A.  In 1995, the East 5 feet of lot A is taken for widening of an adjacent road.  In 1997, your borrower grants a mortgage, the eminent domain case is duly noted, and the East 5 feet are excepted from the legal description.

The legal descriptions on your mortgage and your vesting deed now differ, but the one on the mortgage is accurate.  The deed, on its own, is not.

I'm sure there are many other examples where relying on the vesting deed alone may lead to trouble.