It may not be if, for example, the application is so important to the customer's business that it can not tolerate any sustained period of poor performance. (slaw.ca)
It's particularly dangerous because it causes investors to buy after periods of strong performance (when valuations are high and expected returns low) and sell after periods of poor performance (when valuations are low and expected returns high). (etf.com)
I would rather be somewhat more diversified among international, small - cap and value stocks in case large US growth stocks have an extended period of poor performance. (kevinoninvesting.com)