[czr80]

So, the textbook answer people will give you is risk diversification - even if you believe in the company, why keep all your eggs in one basket?

While there is some truth to this, there is also this: https://en.wikipedia.org/wiki/The_Market_for_Lemons

Basically, the risk is that people most likely to want to sell are those with some suspicion that there is a problem, buyers are therefore suspicious and demand a discount, this in turn drives out people with stock in good companies since they don't want to sell at a big discount, and the market collapses...