Originally posted by: Logic No Jutsu
It would seem that Kakashi chooses to keep his Sharingnan covered with his forehead protector (i.e. a hunk of metal) to keep it from working - no use, no chakra drain, I guess. The same goes for the Uchiha clan -- except they have access to the off button.
Which brings up the topic of Bloodline Limits. I guess a good analogy might be... uh... baldness! Depending on your family lineage, you may or may not go bald in later years. You might keep some of your hair. Depending on the strength of your geans in any particular area, the stroner or weaker you are. A half-breed Uchiha might have the Sharingnan, but probably won't be able to unlock its full potential. Kakashi did the equivalent of buying Rogaine.
If a Hyuuga and an Uchiha got married, you'd probably end up with.... a Uchiuga.