Many people believe that joseki are only played in the corners and at the beginning of a game. But frequently large sections along the sides are involved, and although most joseki are played at the beginning, there are standard sequences that we can refer to as middle-game joseki. As joseki appear in almost every game, this implies the exchanges are even. However, a word of warning is perhaps necessary here. People often play joseki in the belief that the outcome will work out evenly.
Nothing is further from the truth.
And, of course, that is the reason why there are so many joseki. I personally believe there are more than 10,000. There is a saying that the more joseki you study the weaker you get. The reason is obvious. People play joseki without questioning whether or not it applies to the situation at hand.