Why the fascination?...short answer...because they can. People can load, buy, and shoot whatever they want, and that's fine. It's a projectile coming out of the barrel of a gun, and it's going to have some impact on some level. However, I generally agree with the OP's thread comments in light of debate on how effective those "mid-weight" bullets are.
I reload and IMO the best...not only...application of 300BO is suppressed, subsonic mode. You can use all your AR15 gear and components to get a really quiet, accurate, rifle round up to the 100-125 yard level...generally speaking. You can buy lots of off-the-shelf ammo, reload with an unbelievable amount of excellent 30-cal bullets with little powder, and keep it in a very short barrel. If you don't want to do the NFA, build it in pistol format. That's what I did.
I'll be honest. I resisted the concept of 300BO for quite awhile. Getting a suppressor convinced me of the wisdom of the 300BO round. Even though the suppressor allowed me to shoot my 5.56, .223, .243, 9mm, and .308 guns without ear protection, it was a little disappointing compared to my expectations. Once I shot a friend's 300BO AR with my silencer, I was sold. I live in the country, and this weekend I stepped out my back door, under a metal patio cover and shot a rabbit about 40 yards away with my 9.5" 300BO pistol equipped with a Shockwave Blade. We're overrun with rabbits this year. Even shooting under conditions that can make a suppressed, supersonic bullet too loud under that close, metal roof, the heavy, subsonic bullet, suppressed report was nothing more than a hand clap...with about as much of it coming from the action sound of the AR. Afterwards I closed the adjustable gas block to stop cycling for an experiment and was rewarded with even quieter operation. I see why some like bolt action 300BO, even that's not my cup-o-tea.
For me personally, the holy grail of the 300BO is subsonic, heavy bullet, suppressed. I think there are slightly better AR15 compatible calibers when one wants to go supersonic, though personal choices and pursuits are just that...personal.