A cap-weighted index tracker/etf may be cheap but it does create significant concentration risk. Looking at the MSCI World Index, 55% is allocated to the US at a country level and 21% is allocated to financials at a sector level. This, in my opinion, is not adequate diversification. In addition, on a valuation basis the US market (as represented by the S&P 500) is relatively overvalued to other markets, with a CAPE of 25.6 - it's highest level since 2008. If you are purely after beta equity exposure, I think that a non-cap weighted etf such as those which replicate the MSCI World Equally Weighted Index reduces concentration risk and provides exposure to small-cap and value premia, two well known premia that are rewarded over the long-term.