Soyuz 5 and Soyuz TMA-11 survived reentry with an attached Service Module, which simply burned off. There have been no accidents where the Orbital Module has remained attached. What would be the outcome of such an accident? Would the module burn off?
The orbital module was detached in order to make the flight plan easier to calculate i.e. where you're going to land and by not causing any unexpected violent forces. If the orbital module remained attached all through reentry this would probably cause un-expected issues in the recovery if it gets stuck on anything and won't come off. Saying that however it is extremely unlikely that the module doesn't get burnt off as it was just not designed for the forces experienced during reentry. Structural failure of the orbital module during reentry is still probably not a good idea.