Two things, one being ULF movie bass and the second being bass in produced music.
I'm aware it has been discussed somewhat.
1. How/why do the sound engineers working on a project even add these frequencies when mixing theaters don't have the capability?
I don't think they use walls of infra-woofers in mixing theaters nor do they use rotary designs (???). The sounds in the samples library probably contain these frequencies, but even then, I don't see how mix engineers would know how loud to make these frequencies in the mix without being to produce them acoustically. I can see their use being intended for tactile transducers, but not for that rare home theater enthusiast who is actually able to produce them as sound waves.
2. Based on the previous, is flat response below generally accepted hearing limits something to strive for, and if so, why?
I'm aware it has been discussed somewhat.
1. How/why do the sound engineers working on a project even add these frequencies when mixing theaters don't have the capability?

2. Based on the previous, is flat response below generally accepted hearing limits something to strive for, and if so, why?