Actually I have some idea what cpl filter is use for, but wanna know whether ppl will leave it on the lens for general shot? Because I thought of leaving it on as a lens protector and as well as use it when I need it since there are ppl saying that it does not underexpose alot when adjust to the lightest.
filter, as the name imply, it filters away the light.
and all filters will degrade the image quality more or less,
so we only use filters when it needed.
polarizing filter is only working on certain condition, so when there is no effects at all or you don't want to cut away the reflection, why use polarizing filter when it is useless?
beside, it filters away 2.5 stops of light, that is consider a lot, especially in low light situation why risk your self using a slower shutter speed, or make your flash work extra hard for noting?
not to forget it also make your viewfinder darker, if it is at outdoor, you still able to see clearly, if it is indoor, you may have hard time to compose your shots, your camera may have problem to do auto focus as well.
so, it is not advisable to leave CPL filter on at all time, all filters should be use only when necessary.