I think this is a very selective procedure which involves just more than assessing long taxi.
As much I love to comfort the pax, I also think with the current world affairs, where anything can go wrong the smart pax won't hesitate to sue the airline and take that opportunity if an incident occurs on board primarily because seat belts off is not a part of the procedure to begin with.
The heightened security measures taken by various agencies is a sign of how aviation is under threats. Any suspicious activity will go undetected in such a seat belt off scenario.
I hope The captain who probably switched off the seat belts must have been really really aware of the profile of passengers he is dealing with.
I would rather have seat belts ON and with a PA ensure, pax are allowed to go to the restroom, and crew is aware of the pax leaving seats.
Specific to wide body and full flights, dealing with 300+ pax in a seat belt off situation, can be tricky I can only hope that the clueless pax returns to the seat on time, esp after seat belt being put to ON WELL IN TIME BY PILOT.
The seat belt ON is the only legal thing which can keep a pilots tail 'clear' in situations described above.