Most of the people who are with the opinion of selling all the bitcoins are the ones who havent recognized the true potential of bitcoin or who are less than average risk takers. I would say dont hold your bitcoins if you are not confident about it, there is no point of investing without having faith. Bitcoin is a revolutionary technology and has great potential and thus I am very interested to be a part of it from the beginning but thats my opinion and I cant force it on someone else.
What you said is true that we need to have faith in the potential of bitcoins as it will grow much higher in future but if price goes higher in recent times and if its giving you profits then its good to sell a part of it.