Prediction is the core of bitcoin market. Without prediction, entire market is bound to fail. We invest in bitcoins because we believe that the value of bitcoin will be high in future. That's one kind of prediction.
I agree with this statement, I also want to add that you need to specify what kind of investor you are talking about because if a trader dont watch the market it will be non-sense to be a trader, lets say that your talking about long term investors, for me they still need to look whats happening to the market though it hurts them when its on deep because it is important that you know what's happening in order for the investor to make action when there's a big deep they can buy more.
If you conduct a proper study before investment then you must trust your investment.