[pwaring]

The problem with shorting is that you can make the right call broadly, but time it wrong. I think AAPL is overvalued, but I don't know when (or even if) the market will come to the same conclusion.

It costs money to keep short positions open - there are several examples of people who exited short positions in the subprime-mortgage bond market just before the credit crunch. They were convinced that the market was going to crash, but they didn't know exactly when.