here is a huge reason that a person may not want to hold much (if any) stock in the company they work for: diversification. In other words, it is bad to put all your eggs in one basket. Let's say your company goes bankrupt. That means you no longer have a job AND your entire retirement savings is gone (if it was invested in company stock because a bankrupt company's stock is worthless). Unfortunately, many employees put all their retirement into company stock because they don't know any better. They are doing this against the most basic, text-book premise of investing and they don't know it. This is why you hear all those stories of people at Enron having lost all their retirement money. That is the risk you take when you are not diversified.
Companies encourage their employees to buy their stock because they believe it gives the employees an extra incentive to work harder. In other words, if you work hard and do your part to make the company more profitable, the stock price will increase and your retirement savings will increase.
It is a terrible idea to have any more than about 10% if any of your retirement savings in your company stock.