A plastic faceshield can help protect your whole face in a crash. It also protects you from wind, dust, rain, insects, and stones thrown up from cars ahead. These things are distracting and can be painful. If you have to deal with these problems, you can't devote your full attention to the road.
Goggles can protect your eyes from all these things, though they won't protect the rest of your face like a faceshield does. A windshield is no substitute for a faceshield or goggles. Most windshields will not protect your eyes from wind. Neither will eyeglasses or sunglasses. Glasses won't keep your eyes from watering, and they might blow off when you turn your head while riding.
To be effective, eye or face protection must:
- Be free from scratches.
- Be made of material that does not shatter.
- Give a clear view to either side.
- Fasten securely, so it cannot be blown off.
- Allow air to pass through to reduce fogging.
- Allow enough room for eyeglasses or sunglasses if needed.
Tinted eye protection should not be worn at night or any other time when little light is available.