Kakashi is known as "the copy cat ninja". The Ninjas from the other countries call Kakashi the Copy Cat Ninja because of his Sharingan and how he uses it for his advantage (Copying moves). But he does not use his power to copy very often (or at least not anymore). It is only shown around 3-4 times that he actually copies and uses it against somebody.
Why does he not use this power more often?
What everyone was saying was partially correct. The Sharingan is a kekkei-genkai made for the Uchiha Clan. Since Kakashi isn't native to this clan, he's only able to use his Sharingan after once in awhile. As said before, he used it in the beginning of the series and was out for about a week or two from using too much chakra.
Consumption of all of your chakra will cause you to die.
Now with his Mangekyo Sharingan, he cannot use it much. The example that comes quickly to my head is when Kakashi and Naruto were chasing Deidara during the Gaara Retrieval arc. Kakashi specifically quoted that, "We have to end this soon. I can only activate the Mangekyo probably one more time."
In the current manga, he can only keep up with using his Mangekyo Sharingan with the help from the Kyuubi's aura/chakra.
It consumes way too much chakra for him.
All the comments are correct but the truth is that his body isn't built for Sharingan. He uses it because it was implanted as a gift from a dying friend but there's more to Kekkai Genkai than just the gift. It's sort of like the Hyuga clan with the Byukaggan. If you notice there's a network of veins that support their eyes. If their eyes were stolen it's likely this network of veins wouldn't exist in the new user and therefore might strain the user's body.
In the same way Kakashi's body isn't built for extended Sharingan use. Sure, he can use it more than someone who had it newly implanted but he's not an Uchiha. Where Sasuke can use it for very extended periods of time and even strain it Kakashi can not.
The answer might be even simpler than we though. Kakashi is the copy ninja, he is the one who has copied 1,000 jutsus. Why doesn't he used the copy ability of sharingan? Here's why.
He has copied the jutsu already, thus no need for him to copy it again.
The jutsu is not copy-able, such as the Hyouton Kekkei Genkai.
Even if he copied it, he can't use it due to not having enough chakra to perform it.
He can copy it but the jutsu is not worthy of being copied.
Actually Kakashi copies techniques but not everything because only few of them are compatible. Hence he could copy 1000 techniques and not show it because there are no perfect enemy he could use his techniques on.
You have a series where ninja use natural elements as base for techniques that can be performed to destroy villages...your only question is why Kakashi doesn't copy his opponents moves? I think, its not that significant at the moment...also to think about it, most of Kakashi opponents were Akatsuki members, who's each abilities are very unique, probably cant even be copied, apart from any element base techniques. Then again, I cannot remember if Kakashi can copy non element jutsus. Ya dig?
I completely agree that Kakashi should have used his copy moves more. He used it like once. It has nothing to do with how the sharigan takes up chakra, because he already used the sharigan to copy base elemental justsu. So he should use the jutsus he has copied, which don't take as much chakra as his sharigan and mangekyo sharigan.
The techniques he copied from Zabuza, like water sphere, would have been very helpful in the was when dealing with few opponents.
I think it is either the legend of the copy ninja wasn't as great as we thought or the laziness of the writer.
