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I think your assessment is relatively concise: most of the value can be had by simply holding ETH. All investment returns from The DAO must be measured in terms of ETH. It is like investing in Bitcoin mining equipment. You must earn more Bitcoin from mining than you spent to buy the equipment for it to be profitable.

Assuming the DAO invests in some killer startup that builds on ETH then ETH will likely benefit the most which makes earning returns above and beyond ETH gains very hard.