Assuming that squinting is to produce greater depth of field vision, like stopping down a lens on a camera, the image captured by the eye definitely may not be clear. For constant amount of light, the depth of field should increase as the aperature becomes smaller, but the resolution decreases because less light actually makes it into the eye. In manual photography where depth of field is needed, to get both depth of field and proper exposure and resolution, shutter time is increased. On the automated cameras such as some (maybe all?) of the Canon EOS and similar series when using a flash, shutter speed can remain the same, the aperature small, and the camera actually boosts the flash. So instead of compensating with time, the camera compensates by increasing the amount of light.
For grins, try this low light test. Go into a darkened area where with normal vision, you can it is difficult to tell what objects are. Now squint. By reducing the light getting into the eye, you will either lose the objects all together, lose the ability to recognize them, etc. Squinting in low light situations can definitely have a negative impact on your ability to make out and identify objects and people.
except for windshields cause paralax....
