I am talking about a residential DI system, but the math is the same for commercial. Many of the little mini DI systems (We make them here) use a BB filter cartridge. The have a very finite capacity that is easily calculable. In general a 1 ft. DI tank would treat about 125 gallons of water before the media becomes exhausted, and since a BB filter has about 1/6th of a cubc foot of media, you can assume the filter would treat approximately 20 gallons of water before exhaustion. Not exactly efficient or cost effective. If you were to take the same water, run it through an RO and then through the same 1 cu. ft tank, you could expect about 5000 gallons before exhaustion. When you figure the cost of resin at about $250+ per ft3, the cost per gallon of the BB (not including your initial cost of the equipment) to be in excess of $10 per gallon. It would make more sense to buy purified bottles of water from the store to rinse your car off with.
With an RO, your cost drops less than 5 cents per gallon, and it could be far lower if you used a high pressure, properly designed high rejection RO.