Cost is certainly an issue. Last I checked, the STC kits for both BDS and Osborne tip tanks were around $10,000. That does not include installation, probably another $3000 to $5000. Since the fuselage tank is probably on the aircraft type certificate data sheet as a factory option, installation cost would be minimal. My big issue with a fuselage tank would be fuel in the cabin. In the event of an accident, that could be a very bad thing...