At usual propeller speeds, with a usual number of blades (5 or less), the airflow efficiency saving of ducting is less than the extra weight, drag and complexity of adding the duct.
As the number of blades and tip speeds increase the advantage of ducting is clearer. However even here the advantages of removing the duct and instead using clever blade design have been thought about (google "NASA/GE Unducted Fan").
Last edited by Dont Hang Up; 30th Jun 2014 at 11:09.