The biggest reason for owning an index fund like the SPDR S&P 500 ETF Trust (NYSEMKT: SPY) or the Vanguard S&P 500 ETF (NYSEMKT: VOO) is always sound. That is, by owning a cross-section of the market's major companies, you get lots of diversification. That doesn't mean your position won't occasionally lose ground, since stocks tend to move higher or lower as a herd. If you're patient, though, the strategy pays off because you're plugged into the market's inherent long-term bullishness. But every now and ...