I'm not saying it's always good to let it ride. I'm saying that it's not always good to take a profit just for the sake of taking a profit. You should have a reason (other than just fear of losing your gains) and a plan for what to do with the proceeds when you get out. I'd be willing to bet that the majority of those people who lost money on gold were simply speculating when they got in and continued to speculate while their investment sank lower. Pure speculation is a bad investment strategy. You'll lose just as many as you win... perhaps more if you let your emotions get the better of you.
Trading is like a casino. Most people think they know what they are doing there, but the majority of them don't and they lose their shirts.
To continue with the analogy, if you are an expert blackjack player and are on a roll, is it always wise to leave that table when you are up? Sure, if you are leaving the casino and never coming back. But if you get up with a profit only to walk over to the craps table, was that necessarily wise?
This is why I'm tired of hearing people say "it's never a bad thing to take profit". That statement is only true if the game is completely over. For most of us, it is not.
Whether you are an investor or trader, the smart thing is to figure out the best move going forward with very little regard for the past. (Gain/loss impacts on taxes should be considered.)
It's not true that it is never a bad thing to take profits. Sometimes it is bad. It all depends on what you do next with those profits. There are a lot of potentially foolish things that one can do that are worse than staying put. Of course, sometimes it is good to take profits. But it should never be done purely for the sake of selling higher than what you bought at. It should be done after you do your DD and discover a better plan for your allocation going forward.