Well, like I said, it's all a tradeoff...
But... the stock 3-point system is known to be a coherently-designed safety system. The stock seats, belts, body, etc work together as a system that's seen fairly rigorous design and testing.
A fully-caged race-car with 6-points and helmets and HANS and heavy rollbar padding (and everything else: window nets? footwell padding? fire system? etc) is also a well-designed system that will protect you even better, but has some sacrifices (cost, more difficult entry/exit, visibility reductions). Still, totally worth it on a track car.
What sucks is the middle-ground.
On the street you can't do a full safety setup like that because (a) it's completely impractical and probably illegal in your jurisdiction depending how the seatbelt (etc) laws are written and (b) it's not at all safe without a helmet on (cage bars impacting your skull even with padding in low-speed incidents), and you need better visibility than a helmet (+cage, +locked-down shoulders) provides in order to safely navigate street traffic, unlike on a racetrack with other like-minded drivers and course-workers, etc.
So when you've got a street-driven car that you also want to use in track events or autocross, it's tempting to add a few racecar safety features while ignoring the rest on the idea that some is better than none. Some of these combinations of partial safety upgrades are more-dangerous than just staying bone stock.
The two glaring examples that get repeated over and over (and have already been hit in this thread): 4-points (other than the Schroth ASM) due to seat restrictions -> no anti-sub protection in an impact, allowing your lap belt to crush your internals, and shoulder harnesses without a rollbar over your head -> paralysis or death from the car crushing your spine because your body can't move out of the way in a rollover. In incidents where those factors come into play, the driver would've been better off with the stock safety system than with the half-upgrades.
In the end it's up to you where you want to make tradeoffs and how you evaluate risk, but at least make an informed decision rather than a random one
