The only benefit I can find for merging is when building cargo bays.
According to the specs for cargo blocks, there is an unusable wall thickness of 2.5m for each individual block. Merging tells the game it is one block instead of multiple blocks. ( source cited at bottom )
I found this has giant net effects when building factories. I tried this with other blocks like hanger, engine, and crew quarters to no avail
If anyone knows of any please let me know.
cargo bay source