It is always a good idea to install a backup system, even if you don't use it most of the time.
It also depends on your passive design. Air flow through a home is important to dissipate moisture buildup. If you truly don't need an HVAC system as a backup, maybe think about just installing an air-recycling system to balance the temperature and the stagnant air in the home.
For more information:
You should also read Daniel Glickman's Ask A Pro Q&A, "Should we replace our PVC-lined flexible heating ducts with another type of ductwork?"