I mean.. most people that do 2 person parties are probably not level 14. they are probably level 18+
At that point it's really not unresonable that you kill them, also from my experience, most people don't just 'Man fight" them, they abuse their weaknesses that they have an kill them while their disabled.
Even still a level 20 character could totally go toe to toe with a pit fiend. They are strong, but they aren't demi-god level creatures. I never personally found myself feeling "Pulled out" of the experience going to Perfidus and aiding people fighting the monsters there, but I guess to each their own.