Many folks new to geo are often suspicious of realiability of the "new" heat pump technology. One of the manifestations of this suspicion is the frequent requests to leave fossil units in service with new geo systems. 
A family friend who added an addition and geo to his home this year has multiple wood stoves that he uses. He wanted to leave his propane furnace in service as well.
Those of us who install geo need to do a good job of ferreting out the reason folks want multiple heating systems in place.
The first question I ask is why. In our area we have many deplorably aging electric grids that may suffer power loss for days at a time. There fore some folks are concerned about extended power loss. While this may be important for some retro jobs,  new construction homes are so well insulated, in some cases, that you can heat them with a match (or perhaps a few portable electric heaters or a propane fireplace).
The second question I ask is what they use for back-up heat with their existing furnace. Since there is generally more to break on a fossil furnace, they are more likely to leave a customer in the cold. Futhermore, as we put auxiliary heaters on every geo we install, my customers already have back-up heat built in.
In spite of the extremely laborious installation, repairs on geo systems are by and large no more complicated than conventional equipment (and less frequent), so end consumers who insist on fossil back-up with geo and an auxiliary electric heater are suffering from a lack of confidence in our product or our competance. They also may pay more to condition their home as electric coils do not always fit with a fossil furnace and A-coil (and gas/oil furnaces can not operate at the same time as this sort of geo system).
I still have the occasional split system install because of the electricity concern, but most customers with a little extra education determine that this is uneccesary. My friend decided to purchase the larger generator so he never has less than 2 sources of heat, wood and geo. At least he trusted me enough not to insist on 4 heating systems (geo, wood, electric and propane). :)

The Refigerator is a good analogy. No one questions how the magic refigerator works, but the process is exactly the same, just on a larger scale.  that's usually where I begin when I'm trying to explain Geo systems to someone.