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Half the greatness of asset allocation ETFs is that they're a cheap, easily accessible and effective way to invest. The other half is that these products are well-used. Over $25-billion has flowed into asset allocation funds since Vanguard popularized them six years ago. In a recent strategy note, the ETF team at TD Securities said the amount of money in these funds "consistently continues to grow like a weed each month." Asset allocation ETFs are a fund-of-fund product where an ETF company bundles six or so of its stock and bond funds into a single portfolio that offers periodic rebalancing…