Well, for one, if you have a solid idea that the market is going to crash, and that you're going to soon not be able to invest more, and that in fact you're going to have to draw down your investments to survive, then now is a better time to sell than later.

That's a lot of ifs, but that's precisely what we're discussing - how best to behave in different circumstances. "Always hold" isn't the best answer in all circumstances, despite what many in this thread are saying.