This opens wide the doors to market manipulation. The Bitcoin futures market makers can easily see how many contracts have been sold or bought at any given moment. When the number of bought contracts substantially exceeds sold contracts, market makers can sell real bitcoins at exchanges like Bitfinex, thus moving the price down and leaving traders with losses, and vice versa.
Such manipulation could make sense if there is enough volume in the Bitcoin futures market while the price can be easily moved in the required direction. This explains how Bitcoin futures can affect Bitcoin prices, though in a somewhat convoluted or even controversial way, while the number of open contracts can then be used as a contrary indicator of sorts
Some additions may be appropriate
Manipulation of this kind is certainly possible but not right now. It would make sense if open interest (i.e. the number of futures contracts outstanding) was comparable to volumes traded at major Bitcoin exchanges. As far as I know, open interest is pretty inconsequential so far to make this manipulation economically feasible. You would have to risk too much to gain too little. On the other hand, this explains why seasoned traders will likely stay away from cash-settled Bitcoin futures in the future (pardon the pun). They definitely know this and similar tricks and won't let anyone lay hands on their precious dollars