Although slightly to the concept of DC bronze collecting, I want to address your point about why I tend to only slab 100 pagers that I feel are 9.4 or better. It's simple economics because I do tend to upgrade whenever fiscally possible. Therefore, I have to be aware of the potential resale value of some of the books that I have upgraded.
In the past, when CGC first started and I was grading basically all my nice Marvels for resale, I wasn't that concerned about the 9.4 threshold, as I was mainly grading Silver Age books bought long ago so I wasn't worried about getting my money back. The CGC market was much hotter to the lesser grade books a couple of years ago. It has slowed considerably so that's why I have to be so picky. The fact that I am slabbing DCs mainly doesn't help, as most of the buyers are Marvel fans.
Hope this explains my rationale.