If you hold gold for 5 years you'll have to pay to store it, and forgo 5 years of interest, typically based on whatever T bills yield at the time.
The difference between the current cash price and the dec 2015 future is about 14%, so it would be like sticking your money into a five year (zero coupon) bond that only pays out 14% extra at the maturity - Well under what you can get on most treasuries, and highly illiquid to boot.
A more interesting trade would be to find someone due to deliver cotton to you next year, who is mug enough to deliver it to you right now for little extra charge.
Imagine getting hold of cash cotton at half the price you can sell it for the front month future at....
Of course by 2013 there isn't a shortage still anticipated, otherwise everyone stockpiling the physical is going to take an automatic hair cut. It is failing to make delivery that is propping up the front month prices I believe - a situation that won't last forever, lest it be possible to buy the Dec 2013 future at around 90.00 and watch it slowly appreciate to 180 assuming the price stays roughly where it is for the duration. If it was seen as "likely" then everyone would be doing just that eh?
100% over 3 years is a lot better than 14% over 5 years if one believed such trades were viable.