Scott,
I disagree with your logic. I haven't been to a bar that has enough tables for the 1/17th difference in space allocation to really matter. Basically a place would have to have 17 tables side by side to add a single 7 footer in the space previously occupied by the eight footers. On average each table takes about it's width plus five feet, plus five feet divided by the number of tables in a row. So four eight footers would take 41 feet; four sevens would take 39 feet.
I believe that the reason sevens are more standard is that they will fit in more places (places that are too small for eight footers), are cheaper to produce (making a sturdy seven footer is probably 20% cheaper than making a sturdy eight footer), easier to move and cost less to recover, replace rails, etc..
So, I firmly believe that the shift to sevens is pushed by vendors and manufacturers. Vendors have a much lower overhead in moving, setting up and upkeep. Manufacturers are making more on a seven footer than an eight.