Is there any reason to have ADS-B out that is not based on GPS?
There are still quite a lot of aircraft that do have ADS-B based on inertial navigation position. 

The reason for this is quite simple; they don't have GPS or the GPS is not connected to the transponder. Somewhere during the lifetime of the aircraft the transponder has received an update or has been replaced and is now transmitting ADS-B messages, based on the only position source available to the transponder; the Inertial Navigation System. That is fully legal, but it does not meet the ADS-B requirements of the various ADS-B mandates around the world. 



Is this a danger to the safety of flight when ADS-B IN based systems like ATAS are used? 

No, not at all. ADS-B IN systems do not use the data from these aircraft as the ADS-B messages contain several quality indicators that must meet certain conditions before a track is being displayed in the cockpit. Transponders transmitting INS positions indicate that the position integrity and accuracy is very low. In addition the source integrity level (SIL) will also be set to 0. Each of these indicators will result in the track not being displayed.