MS and EMS is very confusing and not everything is known.
Hell we saw tobi admit during sasuke vs Danzou that not all sharingan stuff has been figured out yet.
Itachi said MS goes into gradual blindness, and it speeds up with the use of it.
Kakashi should be at least slightly blind in that eye already, even with limited uses.
Personally kakashi could very well have EMS, as we dont know all the sharingan secrets.