Though I am far from a pro trader, I know that is not entirely true. There is such a thing as cutting your losses.

Example, if you bought at $600, price is falling and you know it will hit $300 before a bounce, it is better to sell at $500 and buy back closer to floor than hold all the way.

Of course if you don't know/have an idea of where price will go it is better to hold, as there's a large chance you'll miss the knife entirely that way and only make your losses worse. But for a decently competent analyst, cutting your losses and making the best of the situation is better than stubbornly holding throughout and refusing to sell at a loss.