A long term portfolio has to be necessarily long term. Avoid getting in and out of stocks too often. Your core portfolio should change only if you perceive a serious change in the fortunes of the company due to some emerging trend that is likely to disrupt the performance of the company. When you overtrade you need to worry about the impact cost, trading costs, regulatory costs as well as taxes. As a smart long term investor you surely wan to minimize each of these costs to the extent possible since each of these small items can compound smartly over a long time frame.