Sorry, but what you are ignoring rvoll is dirt accumulation in the oil.With today's oils and the tolerances on modern engines, there is absolutely no reason to change your oil at lower intervals than recommended by the manufacturer. This is especially true if you use synthetic. This was not the case a decade or two ago, but is definitely true today. Some synthetics will even go 15-20k miles. I did test my oil at 6900 miles, and it was fine and had at least 20% more life. Changing it more often is just not necessary and increases maintenance costs. I now use a full synthetic in all of my cars and my safety margin is huge. If it weren't for the 10 year warranty and the need to prove maintenance to Kia in case of an engine failure, I'd do it every 10k miles....
Anyone that thinks they can run 15-20k miles is just fooling themselves....unless you have no intention of keeping the vehicle for any length of time. Sure the oil might still have sufficient additives at that interval, but it is also extremely dirty, and that leads to wear.
And what about the oil filter?, just going to leave that in there?