Just thinking is it because drilling that many holes across the block in close proximity would or possible increase stress on the plexi they use, so better to use a plate, just a thought
actually I dont think the three extra ones in the centre will cause additional stress. I was thinking more along the lines if its possible the nickel block is raised higher in areas due to caps, and then it would make the plexi thickness thinner thus prone to more stress. Could be wrong though, but its been a curiosity of mine for a long time. I have always used reference blocks.