The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.NEW YORK ( ETF Expert) -- Not every exchange-traded investment is a sensible core holding. In fact, some may not even work particularly well as trading vehicles. In many instances this is the way that I feel about volatility ETFs/ETNs. Volatility ETNs have been a nightmare for buy-and-holders. Many have seen their principal erode 30%, 40%, even 50%. And yet, the iPath S&P 500 VIX Short-Term Futures ( VXX) has demonstrated utility as a predictive tool. Consider the two worst bear scares in the current March 2009 to May 2012 bull market. The first came in late April 2010. A steep 16% top-to-bottom decline essentially ended in the first week of July 2010. The second came in late July 2011 when the U.S. government failed to agree on a debt ceiling deal and Standard & Poor's downgraded U.S. Treasury debt. The devastating 19.5% top-to-bottom sell-off effectively ended in the first week of October 2011. Now consider the times when the iPath S&P 500 VIX Short-Term Futures definitively broke through its 100-day moving average to the upside. You guessed it... in late April 2010 and July 2011. What about when VXX descended definitively below its 100-day trend line? July 2010 and November 2011. Not too shabby.