Here is a quote from an article I read recently:

People often ask me if one studded tire in front is sufficient. Well, one studded tire in front will help keep you from crashing hard and breaking your collar bone or your pelvis. When the front tire slides, you go down quite fast and without warning. If the rear tire slides you can still go down, but not as fast, and the results are usually not so calamitous, since you usually have enough time to get a foot out of the pedal. But with just a front studded tire, you can still crash. You can still not be able to climb a hill with black ice. You can still not be able to get out of an icy rut. You can still spin on a downhill curve. You can still lose control in a busy intersection with lots of traffic. And you can still spin during hard braking. So, for the life of me, I can't see any reason to use a single studded tire in front, except to save a few dollars.

I do occasionally sell a single tire to someone who's been told by a friend that you only need a front studded tire. Similarly, there are many folks who think you can get away with snow tires just on the drive wheels of an automobile, which leads to cars spinning out of control because the front or rear has so much more traction than the other. Invariably I get a call a week later from the same customer ordering a second tire. He ends up with no cost savings, since he has to pay twice for shipping.

If you are buying tires for riding single track in winter, and you need to save money, there is a way to do it. Get a very aggressive tire for the front, an Extreme 294 or Ice Spiker, and then use an Extreme 120, Mount & Ground or Snow Stud in the rear. You won't get quite as good grip while climbing a steep trail as you would by having aggressive tires front and rear, but if your trails aren't too steep, you should be just fine. The more aggressive front tire will still be there to get you through icy ruts. For the commuter riding paved roads, there really is no alternative to having the W106 or A10 on both wheels. So unless you're riding to work on rail trails, I strongly recommend you use studded tires on both wheels, not just the front.