5 Year Rule

OK, no flames please, but can someone explain the basis of the rule about "buying only if you plan to stay for 5 years or more." I understand the general concept, but don't see how that applies currently. E.g., Suppose X purchased an apartment in December 1, 2006 for lets say 850,000. Five years from then is December 1, 2011. Even though X will have technically held the apt for 5 yrs at that point, the predicted condition of the market by next Dec makes it highly unlikely that X will sell for even 850,000 (I know of course that we cant say this for sure). So in this case it doesnt even matter that X has been there 5 years, still screwed, no? No flames please, I am a beginner here!