The KDS-21 I have only does one final rinse, and so does the new Maytag MDB9000 electronic machine I have. At first I was skeptical of only one final rinse, however, on the KDS-21 I havent noticed any lack of performance. Most machines I have encountered with two final rinses have the first rinse only one timer increment meaning its only a two min rinse or so, very short compared to the final rinse which follows. The RR Maytags generally have two five minute rinses. But the GE and WP machines I have do one short rinse, followed by one final longer rinse. 

I do see the dilemma of not properly rinsing off the detergent. 

What are your thoughts on just a single final rinse as opposed to two? Have you ever noticed any performance issues with only one rinse?

I know this is going back decades, but when KA introduced one final rinse, I noticed precipitate on the dried load.  I would wait for the post wash purge to finish filling with one quart of water and quickly dump in 3 quarts of hot water to make it a rinse. That machine was so insane with stupid ways to use water that I was glad to gift it to my sister and her husband who kept their water heater at 120F and did not know shit about dishwashers' requirements. It did clean better with low temperature water than the bol dw it replaced. Fortunately, they did not take it out when they put their house on the market and the buyers were impressed by seeing it in the kitchen.

BTW: when CU tested the APEX dishwasher in the early 50s, they noted that the first model had just one rinse and left food debris behind while the newer model had two after rinses and did not, so they recommended setting the older model for a second rinse. I know that was a lifetime ago, but it shows the importance of adequate rinsing for soil removal. While new machines don't generally leave soil particles in the tub like the older machines, that ground up stuff is trapped in a filter or some soil separator with detergent . One rinse does not carry away all of that, especially with the ultra low fills in modern machines. It didn't with the one rinse KA I had with a higher fill level.  Soil and detergent residue might not be visible as particles on the dishware, but chemically it is there.