For 80hz, maybe 100hz and below a sub can do the job.
I don't think most 4,5,6" do that. Maybe 8", but then your losing some of the upper mid's the 4,5,6" drivers can do better as they can move quicker. thats how I understand the concept. very generalized.
as far as playback systems, i would think the Eng wants to have the ability of reproducing and checking the full range or as close as possible? only a sub will get that low while releiving the mid driver to concentrate on the mid freq.
so in HR they work as mains and also the "average joe's" system check without leaving the chair...which nowadays has a sub too, imo.
other playback systems, would be whats ever "in fad" at the time.
Ipod, Earpod, Qpod, Car CD with and without subs, HomeTheater with Subs....
and some gold records have been done with Radio Shack Optimus 7's.
so I give up, time to go take my medication and get in my small metal transport device and ziong my way to the dark city of overpopulation...