I'd love to have all the test equipment to see exactly what the stresses & dynamics of a main cap are under operating conditions. However, my thoughts after talking w/ the machinist who prepped my last two blocks are:

1. The girdle is NOT expected to keep the caps from being pushed directly down.
2. The girdle helps add stability by keeping the main caps from "waiving around" due to the various directional loads placed on them.

Think of this like a flag pole that is unsupported at the end where the flag is being moved around by the wind. If the normally unsupported end of the flag pole was also attached to a stationary object, the "waiving around" will be reduced. Yeah, another over-simplified analogy on my part, but IMO it still makes sense.

Therefore, even the low-budget girdles should still add some level of improved bottom end durability, even if not the optimal (re)design of cap / girdle / block integration.