In an emergency stop you weight shifts forward and therefore lifting weight of your back wheel. When you brake with your rear brakes you only get a skidding back wheel and no stopping power. On a shortwheel based recumbent bike its even worse with all the weight on the front wheel you have almost no stopping power at the back wheel at all.

When in slippery conditions the rear brake can be usefull as well as when going downhill. You can then use the front and rear brake at the some time or switch between them to let them cool down.
You can also use the rearbrake to slowdown a little bit but not for coming to a complete stop.

In all other conditions: the front brake is the best option, it gives you the most stopping power (about 80% of the stopping power comes from the front brakes) and the most control over your bike because when your backwheel skids you fall.