The big question is WHEN you buy. If you buy in the middle of a bull run, fail to sell at the top, then the price crashes below your buy price and you're still holding, then you're missing out on the gains that you could have made if you bought later. You'd still be making profits in very long run, most likely, but it shouldn't be that hard to find the bottom or close to the bottom of a cycle. Consider this a slightly advanced for of "buy and hodl".