You're exactly right. Regardless of a blocked pressure cap, most systems won't explode unless something is really on it's last legs.

However, with compressed gases or exhaust gases passing to the cooling system, the readings go right to the sky. And the tiny passage in the coolant cap is not sufficient to keep pressures to a normal level when certain conditions are encountered (heavy throttle, high RPM, or whatever seems to cause the worst pressure in that specific case).

Ford 3.8L are notorious for that. If you've seen a few of them, you've seen leaky heads with gas passing into the cooling system. All it takes is a few minutes running and you can literally watch the coolant bubble in the reserve tank.
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1992 940 wagon, low miles
as well as others.