Fridges and their compressors aren't designed to cool liquids or solids. They are designed to maintain the ambient temperature of the air in the fridge. The reasoning behind this is that it takes less energy to cool the air instead of the objects in the fridge. As long as the fridge maintains the temperature of the air, the objects in the fridge will maintain their temperature.
In the short term, that is an untrue statement. Yes, objects in the fridge might eventually reach equilibrium with the air temperature in the fridge, but the timescale is too long to do us any good.
Say you start with your wort and the air in the fridge both at 65F. As fermentation gets going, since it's an exothermic process, the wort can get 5 to sometimes even 10 degrees warmer than ambient. If your temp controller is measuring the air temp, it'll turn the fridge on just long enough to cool the air back down to 65F and then cut back off. NO way does that get the temp of five gallons of wort down to 65F also. That warm wort will warm the fridge air, the controller will turn the fridge on again, cool the air, and then shut off. Lather, rinse, repeat.
Eventually the wort temp will reach 65F but it'll take days and days for that the happen. By that time, the damage (due to too high fermentation temps) has already been done.
If instead, you're measuring the temperature of the wort itself, when the controller kicks in, it won't cut off until the wort reaches 65F. Yes, it'll run the fridge longer, but it'll get the wort to the temp that you actually want it at. Also, due to it's thermal mass, it'll take much longer for the wort to warm up, thus making it a much longer time before the fridge needs to run again.
So it's not even as good as six of one / half dozen of the other. Measuring the air temp, your wort probably won't be at the desired temp during the time you most care about.