Because Kakashi is not of Uchiha blood, using the Sharingan demands a great deal of chakra. He also cannot deactivate it, and so keeps it covered when not in battle to prevent the chakra drain and the following extended bed rest
I am under the impression that when a Sharingan is transplanted to another host it becomes eternal in Kakashi's case never being able to deactivate the Sharingan it would burn out the eye causing blindeness years ago.
He seems aware of the Mangyenko Sharingan's detoriating affects on the eye because he questioned Itachi about it long before it was revealed to Sasuke about the affects of MS. However the negative affects he has shown was to be hospitalized for 3 days and dieing cause of the chakra drain from using MS, not blindness or detoriating vision
welcome to hear any ideas or disagreements