The IRS does this to help match your rental income to the renter's potential rental expense. They want to make sure you are reporting your income and the renter is claiming a legitimate rental expense deduction.
I'm not sure if missing the renter's SSN will cause a problem with your e-filing (let me know).
If you had to file by mail (or e-file), IRS may question their rental expense deduction, but they won't question your report of rental income.
