It's not the most popular index fund, but maybe it should be in light of the balance it offers.If you're a fan of exchange-traded funds, then you're also likely a fan of index investing. Most ETFs are built to reflect the performance of an index like the S&P 500, after all. Indeed, the world's most-owned exchange-traded fund is the SPDR S&P 500 ETF Trust meant to mirror the world's best-known market barometer.What if, however, you're indexing wrong?The question seems crazy at first. Index investing is the o ...