I find that the deck block plans (plans put out by the actual block company) seem to really push the use of the block - I guess that's good marketing - if you can sell someone (or everyone) who downloads your plans 3x what's needed, then your income just increased marginally.

I believe that the designs are for the worst case scenario soil that you will be building on, if your soil cannot support much weight, it makes sense that you would need many blocks, however, if you're building directly on bedrock, the number of blocks required would be significanly reduced.