Specific reason is that the T/S parameters of the subwoofer will change to unknown values. This will change the enclosure requirements for the subwoofer to an unknown volume, and the entire performance of the subwoofer will change as a result of this shift in the parameters. Also keep in mind that the power handling of a single coil is half that of the driver as a whole (i.e. if it's rated for 400w RMS, a single coil can only handle 200w).
It is possible to do, and if you know how the driver will perform with only a single coil, you can still obtain good performance. Some people do this intentionally and add a potentiometer or resistor across the open coil to allow for an "adjustable" Q (this is for the extremely advanced users only). Ascendant Audio specifically did this with their Atlas series of subwoofers, if you are familiar with it. While what they did is not unique (it can be done with any dual voice coil subwoofer), they were the first that I'm aware of to actively market this feature. But the reason it *worked* for the general user is because Ascendant gave the resulting T/S parameters and enclosure recommendations from using specified resistors across the 2ohm coil.
But in general, and for someone with limited experience, it is not a good idea to attempt.