I spent 30 years in the safety business and my answer was "it depends on the hazard" and that's a lot like what I've heard here in some places.
OSHA does not apply in your private shop! However; here is what they say:
Safety spectacles are intended to shield the wearer's eyes from impact hazards such as flying fragments, objects, large chips, and particles. Workers are required to use eye safety spectacles with side shields when there is a hazard from flying objects.
Face shields are intended to protect the entire face or portions of it from impact hazards such as flying fragments, objects, large chips, and particles. When worn alone, face shields do not protect employees from impact hazards. Use face shields in combination with safety spectacles or goggles,
Source:
https://www.osha.gov/SLTC/etools/eyeandface/ppe/impact.html#spectacles
You notice neither protect against flying chunks like when a bowl breaks apart or slips out of the chuck. If you're turning something large like that, you need to have the guard installed and in use. I just had a good friend who broke every bone in his face because his guard was protecting a shelf in his shop.
Lynne Yamaguchi was seriously injured a few years ago and she uses a police riot helmet along with the lathe guard when she turns bowls now.
I wear prescription glasses and every time I get a new prescription, I get a pair of prescription safety glasses. That's just my choice tho. I like to see what I'm doing and be safe.
When I turn pens, I wear only safety glasses. When I turn other items like small boxes or such I usually throw on the face shield over my glasses. When I turn bowls, the guard comes down and I wear both.