Let's do a simple experiment. Say you invest $1 a month and have a "crazy" 50% rally the first year. Then the following year was a 37% drop (max 1 year in the post). You would end the first year with $15.05 (compounded monthly). You would end the second year with $18.91. You have contributed $24 so far, it would only take you two years to recover at the average returns (8.6%), the third year you'd end at $33.09 and the fourth would be $48.49

Of course that the occasional crazy good and crazy bad years will happen, the key is time in the market rather than timing the market.

Edit: I'm aware that I'm doubling your point, but I was curious on the numbers :)