2) Since it would be implemented as an amendment, the Government might resort to dirty tricks to ignore it after getting the OK for TM's deal.
1) It buries no deal, which is massive. No matter what, no deal would be toast.
2) It adds a veneer of extra legitimacy to the process (rather than eg having a hated deal imposed by 50%+1 of MPs who have been scared/bullied/bribed to vote for it.