Suppose, for example, that you really really liked living in your house, but were strongly convinced it would decline in value. You could go more short than long, very much like overinsuring in the intended effect. These proposed futures would make such an exercise perfectly feasible and legal [which it currently is not!] If you were convinced your $500K home would fall in value to $400K, you could short double the number of futures contracts req'd to hedge a constant value in your home. Home falls to $400K, you cover half your short, pocket $100K, and the combination of home + short futures contract would still maintain a constant $500K value. Kewl?
But seriously...every short position must eventually be liquidated, thus purhased so is essentially bullish.