You never know how long a trend will continue. This is why to maximize profits, you gradually raise you stop order as your position appreciates in value. Once the trend ends, you never know how much it will travel in the opposite direction. People tend to overshoot on both directions.
With an individual stock, you absolutely have to have a stop-loss point, because you never know how far down the stock is going. I remember selling a $100 stock one time and it eventually went to $1. I didn’t have any idea it was going down that far, but what would have happened if I had held on to it? One mistake like that and you can’t come back.”