There are some valid and interesting uses for the "smaller" bigprims that don't cover an entire sim. I'm not sure about this part, but as far as I know, the gigantic multi-sim ones may not have any physical effects on other sims until they cross over, other than maybe appearing in them. The large wall can be used to cut down prims on larger builds, as can the rest of them. The 40m sphere would be great for domes, and I used one to make a custom change to the sky base for my multi-gadget.
I don't know, seems it would be best to cull the gigantic ones that nobody can sensibly use and let the others remain. Sure, maybe they can help griefers, but outside of maybe the multi-sim ones, don't give them anything better that what they have now.