Oddly enough prior to 911 FDs were much less often with only the 2 crew up front. This was down to the fact that there would be regular FD visits from interested passengers, or the jumpseat would be occupied by ATCOs or other aviation related personnel. During my ATC career I spent about 90% of my many flights (as pax) in the FD jumpseat. Sometimes (in certain types) there would even be two of us!