No sanctioning body that does tech for any motorsport approves that sort of down leg on a cage. It only exists because folks buy it and it is easier, not because it works. So no, you don't see them on cages because there is no such thing as a fake cage or a true cage, there are cages and there are fancy CB mounts.
That is correct. And, since neither them, you, or I can predict the scope that will contain our rigs to a specific type of accident, flop, or roll, we have to build around that uncertainty which means you have full protection or you don't. If you don't, then why bother? It is akin to wearing a pair of safety glasses out in the shop when you are grinding that have a 1/2" diameter circle of protection right in front of each eye, that will stop most everything, right?
Yes, the windshield area can use some help but what most miss is that is not an area that typically produces lethal results in a roll. Sure it distorts and gets displaced but if you draw a line from the front of the hood to the top of the B pillar, your head will typically exist just fine with minimal damage in the area under the B pillar.
As for the cleanliness of the angle type down leg versus the tube down leg, I suspect that may be due to not seeing one done reasonably well. Before I quit installing various offerings by the aftermarket for folks, I did this one.
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It may occur to you to wonder why I quit doing that type work since it really isn't terrible looking. I quit for several reasons all of which were a common theme among every client that wanted the installation.
1-when I explained that frame tie ins cost as much to do as everything above the floor, they all declined to do them.
2-Each one explained that they understood they were not getting something safe but their Significant Other would not let them take the kids unless they did something, so something was better than nothing.
Essentially it boiled down to everyone needing to predict the future and thus keep their protection within the scope of that prediction and I won't be a party to that.
Almost lastly, at little illustration of why stanchion down legs are very bad. I used to compete in rock crawling events. I was the spotter. On one obstacle, you had to drive up high on the right side to miss a cone. It wasn't possible and the cone was there to be taken out. My driver didn't want to take it out as was his norm, very competitive. As he drove it up, it flopped to the left but there was a rock on a nearly flat section about the side of a basketball. When the rig landed, the rock hit right on the windshield hinge. From just a flop and low speed, it displaced the entire dash with the "cage" work over to the right about 2" forever ruining any hope or chance of getting a top or doors back on.
If you were to do a similar type flop that was on a narrow section with another drop off that would send you into another roll and that same type rock happened to be below the hinge where it impacted the area where the stanchion is, say right on the lower door hinge, it would fold with a quickness and very little force. So now the down leg that you want to hold the windshield up is full compromised and will not do what you want it to. At this point you may be thinking that perhaps that isn't so bad so I'd encourage you to remember that just inside that stanchion happens to exist the legs that belong to someone you are likely quite fond of. About the last thing you want is some bent angle iron in the same area your legs are as they are flopping around in a mild roll over.
The biggest reason I quit doing installs besides the above is I saw the false sense of security such things offered that may induce one to take more risky lines and speeds than is prudent. I would rather operate with the stock roll bar and mitigate my risks with my behavior.