It depends what you are using the index for. If you investing in a mutual fund that is replicating the S&P 500, for example, it doesn't really matter except fo

It depends what you are using the index for. If you investing in a mutual fund that is replicating the S&P 500, for example, it doesn't really matter except for transaction costs. If you are using the S&P 500 as your baseline (e.g. to decide if an investment strategy is worth the effort) it doesn't matter at all. If you are using the S&P 500 as a metric about the economy... it might matter. If there is frequent turnover in the composition it might indicate that the market is very volatile... (someone with more knowledge can probably tell me if this is true or not).