Well, to those people, I'll point out that a big drop in one year doesn't prevent a further big drop in the following year. That can happen, and the second drop can sometimes be bigger than the first. Of course, sometimes the market will rebound strongly right after a big drop. Check out these returns for the S&P 500 for some examples:
1930 (25%)
1931 (43%)
1937 (35%)
1938 31%
1973 (15%)
1974 (26%)
1975 37%
1976 24%
2000 (9%)
2001 (12%)
2002 (22%)
2003 29%
2004 11%
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