Originally Posted by almanik
It gives your opponent information about you. He can use it and beat you in the future.
What if you reverse it and use the information he thinks he has on you against him?
Example, two times you successfully bluff him out of a pot and show him the bluff both times. He now knows you're capable of a bluff and will be more likely to pick it off in a similar situation if you try it again. You get heads up with him and make the exact same play against him, and he calls this time trying to pick off the bluff, except this time you show him the nuts and hand him the keys to Value City. You get paid off and the information he thought he had against you goes out the window since now your range in that situation is polarized, and he no longer has an advantage.