Yes they do. As J. P. said, "The market will fluctuate". As Zeev pointed out, at any given time, there will be wide variations in relative valuations. Certainly, one can take advantage of those variations to capitalize on the fluctuations.
For the market as a whole, the reason I believe we are far from a valuation low can be seen on:
While we may not get to a P/E of 5.6 (in '32) or 6.6 (in '82), unless "THIS TIME IT IS DIFFERENT", we will get < 10. But as I mentioned before, it doesn't have to occur in one plunge.