It's not a matter of intelligence, it's a matter of the way boats were made and equipped in the past, and human nature. My 1984 Catalina 36 didn't have a sniffer, and I never felt the need to add one. I would usually shut the solenoid off and watch the flame go out, as a matter of habit. My new boat has a Fireboy-Xintex propane/CNG Fume Detector with Solenoid Valve Control, and I said "great! Now I have a sniffer!" It would not have occurred to me to install one. I guess it's the same for most people. Maybe we should conduct a poll on this site.