Re: Who is training IS and why still?
I train IS from the perspective of understanding the whole "cultivate yourself" perspective that martial arts were eventually purported to enable beyond the immediately practical battlefield/dueling/fighting aspects (spent my time in combat sports/arts so already been there, kinda over proving whether I'm tough or not). What's fun is that over time I continue to practice these things as my balance, sensitivity to the connection between myself and another person, and ability to receive/issue power all increase - they translates nicely into practicing aikido waza, putting the gloves on and sparring, or body/body grappling.
I think of timing differently than I used to - I can join better with another person before touching them in a way that I would have considered more "woowoo" stuffs pre-IS training. Then when we do make contact I can be "already arrived" in an initial superior position with regard to their balance (presuming their skills aren't better than mine, or their size/surprise/strength/speed/skills combo doesn't otherwise offset the advantage IS can confer). Same thing at striking range as I can "line up" on someone with minimal movement that can make it harder for them to throw a clean strike with power (again, superior skill/things can overcome this advantage). It's been a while since I've played with weapons, but I see definite overlaps there, too.
As far as raw power, I can hit considerably harder than I could pre-IS, with much smaller movement. I can also hit with more power from weirder angles and with different parts of the body - which has more to do with growing the ability to hit with "all of or as much of your body moving together as one unit as possible" out of any one part of your body (hand, elbow, shoulder, head, etc.) rather than a new hitting "technique" (elbow strike, headbutt, etc.). To me this is much closer to the aikido intent of "atemi" than a strike that intentionally doesn't connect, or presumes uke will move - or conversely adding on pugilism from another system (karate, muay thai) to aikido (not arguing against it, but I'm thinking pure "body strikes" rather than pugilistic technique in this definition).
Grappling gets interesting as well (and I consider many waza of aikido to be a standing jujutsu form of grappling) as your balance and connection with another can give an advantage that makes it feel like you're ahead of their "balance loop" such that you can be immovable along one line of attack, then if the line changes you're ahead of it to capitalize and deflect it into another direction, or counter - without changing much of your outward configuration (again, this advantage can be offset somewhat by other attributes). This I think (along with hitting) gets into the realm of how it "feels different" and "must be felt" for people to understand if and why they are or are not "already doing that".
But I think the genuine seeker will be looking at these things with healthy skepticism and an open mind, while hopefully being smart enough to clinically assess encounters and things seen/learned so that they can make their own choices and eventually own their own progress.
One of the truths I believe around this type of study is that there's a huge amount of independent work that the individual must be self-accountable for - physically and mentally. On one level, this stuff is basically physical conditioning in a different kind of way with a different purpose and desirable outcome than is common in western sports practices, but still rooted in hard work, hours of practice and an unending drive to self-analyze, self-criticize and self-correct. On another level, the mental work that accompanies the physical exertion is exhausting, prone to misdirection, self-perception issues and generally a pain in the tookus.
And like others have said, it's opened my eyes to the foundational "stuff" that goes into all martial arts at their roots, in varying levels of sophistication and purpose, which leads back to self-cultivation, personal ownership and empowerment - which has been a huge appeal for me in years of martial arts study. The pursuit of these things then has an enabling effect across a multitude of other endeavors.
Plus, it's a lot of fun.