I would consider free longing and turnout to blow off steam to be two different things.
I can only free longe in a fairly contained smaller round pen because otherwise there is no containment and maresy just disappears. In a round pen we longe just like on a line, w/t/c halt and turn, on voice cues and a shake of a whip if needed. It isn’t the place for run n buck.
Since maresy lives in a stall with a small runout, I do also offer her turnout in our large turnout arena which has good footing, sometimes alone and sometimes with a buddy. If she needs to run n buck and blow off steam, she can do it there. She’s been on this routine for seven years and hasn’t hurt herself, and I think the buck and bolt is in fact good exercise and I am unlikely to ever ride her flat out that fast! She is however noticeably less energetic the day after.
I don’t like to free longe in the round pen too often as I find the sand footing and the small circumference cause her hind legs to stock up if go in there and canter more than once a week. I don’t indeed longe her a lot, though we do lateral in hand work at bit.
We ride pretty much every day, and my preference is to ride. But if she is getting noticeably bottled up and hyper under saddle, a good run n buck turn out sessions gets the wiggles out.
I realize that people with high value horses may not feel comfortable seeing them running loose full speed, but I feel that over the years this has helped her agility and condition, and I can’t imagine what it would be like for a horse that never gets to run full blast.
When I was a teen i kept my horse stalled but I also took her for full out full blast all rockets firing gallops on safe straight stretches, where I did nothing to interfere with her and just let her blast on a loose rein. She also stayed very fit and healthy. I don’t have the place or perhaps more accurately the courage to do this in middle age on my current horse.