It's a common misconception. People tend to buy at a given value and when this value starts to decrease they tell themselves "hey, at least i have the same number of tokes I had before the crash, nothing is lost as long as I hold." If the fundamentals of the coin are broken and it will continue to lose value, at some point you'll be holding something that nobody will want to buy from you. Something that you won't be able to exchange for any physical item and denying it won't help you then.

With BTC you can wait until it recovers, because it's a great coin that won't suddenly lose 90% of its value, but don't be a fool to think this can't happen to some altcoins.