The reason blocks work so much better is, they do not give. When you place a tub upside down on your spacer and place the shelf on the tub, pressure can cause the tub to "squish" down. This can make your tubs tight if you are not careful. Plus, if you are building several racks with the same tubs, it makes thing so much faster. If you have access to finish nailers, you can assemble a 10 space rack in about 10 minutes. Just be sure to screw everything after it's all nailed together and make sure you nail stright in to avoid blowouts.