I’m in this group you’re talking about. I have not taken a decompression course, I routinely exceed my computers decompression limits, and I do not calculate my remaining air. I would probably not behave this way if I was diving square profile tables!
It seems safe-ish though. When I hit deco at depth I start heading up, and the deco goes away as I do. I slowly ride my computers warning up to the surface, do my non-mandatory safety stop with 1000#, and it and I are both happy on the surface. If I misjudged things a bit I’ve had two or three minutes of real deco at 10’, but that’s more of an extended safety stop than an underwater emergency, right? I have never been close to a situation where I did not have enough gas to make it back up, even if I can’t quantify how close.
Safety is only one of the factors to be optimized here: I dive for fun and would be much safer just staying in bed all day. It’s my (limited!) understanding that spending most of the dive slowly ascending up a wall or wreck is easier on my body than a square up-and-down profile anyway. Diving like this allows me to cover a lot of different terrain, see a lot of different things, and decide what to do based on what I see down there rather than following a fixed plan. The fun / danger ratio seems high, even if I can’t quantify the fun either. Do I just not know what I don’t know? Are there things I haven’t considered? Would I learn those things in a deco course?