Disclaimer - whatever is the antonym of "gold bug," that's me.
By most measures, gold is in speculative bubble territory right now. I find it curious that the correlation is high between the rising price of gold and those who want to buy in. If a Ford Taurus were suddenly $50K, would you be more or less inclined to want one?
Many (not all, maybe most) planners will advise a mix of stocks and bonds for diversification. At a certain size portfolio, one would be sure the stocks are including foreign companies, and one might also add real estate in the form of REITs. One can do very well just with this type of approach.
That said, if you created a mix of commodities, say 10, and each year rebalanced, keeping the 10% level for each (in your commodity 'fund') you might actually get a decent return over time, as the volatility of each would provide a return, the rebalancing having the effect of buying low/selling high, and as a whole it would be uncorrelated to stocks or bonds. This is one premise of managed futures. It's also not for most investors as most lack the discipline to succeed at capturing the benefit of such an approach.
Your ETFs listed contained agriculture as well as metal. In the long term, my lifetime, say, as I am 48, food has gotten cheaper. If you could eliminate the oddity of the cost of futures contracts and storage and simply bet on the price, long term it costs fewer man-hours worked to buy food. For this observation, I don't consider it a long term investment, but only to be used, if at all, as a component of the managed futures approach I mentioned.
The short answer to your question is "for most people, none."
Edit - when I wrote this answer, gold was at $1770. Now, April 2015, it's at $1200. The S&P at the time was 1200, vs 2100 today. Gold down over 30%, S&P up 75% (plus another 7 or so for dividends.)