Working for company X and having a substantial portion of your retirement plan in company X is simply exposing yourself to too much risk, because the company is both your employer and the source of your retirement income. So if something goes wrong, you lose both your job and your retirement plan.
We make too much out of past performance, and it's very misleading to investors. It causes them to move money around. They buy a fund that's hot and then it turns cold as all hot funds eventually do. And then they get out. Well, buying at the high and selling at the low isn't going to leave you a satisfied shareholder, right?