I find this interesting, because I can see no reasonable reason that the rear should fail before the front, especially, since the rear units in all probability saw considerably less service time than the front units. There are two distinct parts of each unit, no matter where they are placed. In the heating system, you have the electrical fan, and the heater core. Heater cores do leak, but they are easily repaired, and removing and having the unit reconditioned isn't a big deal with either. It is more of a labor issue to get them out than anything else. The heater cores on my 1963 Chrysler are both original, and both are still functioning fine. If the electrical fan fails, it also can be repaired or replaced. Finding a replacement can be more of a task, but it will not put the car out of commission. On the A/C side of things, it gets somewhat more complicated with the components of each side. If you change all the hose seals, and service the compressor unit then the unit should be fine for many years to come. Most times that an A/C unit fails, it is either a seal failure or the compressor that is the cause. The reason for most seal failures is that the unit isn't run in the winter, causing the seals to become dry and shrink. If the compressor should fail, it doesn't matter, since it will effect both front and rear units. Compressor failures are usually a result of loss of refrigerant because of a seal failure. If you want to consider that if you remove the rear half of the system, that you lessen the chance of failure by 50%, that I would agree with, but you still haven't changed the possible failure rate of the front unit.