There's not anything wrong with simple drain and refills, and you get anywhere from a good third to half or more of your fluid changed this way. In many cases however, there is no drain plug and the pan must be pulled down to drain the fluid. In that situation especially, I like to do a drain, filter change, and refill/flush. Lots of times I actually open up a cooler line to pump out most of the fluid in the pan to make it easier to drop down for servicing. Also, if the fluid is really degraded, a cooler line flush is the quickest and least expensive way to replace all the old fluid.
If you have a drain plug, it makes it rather easy to just drain out the 3-7 qts in the pan every so often to keep the fluid in good shape. It just does not seem like the smart thing to do with a ten dollar/qt premium fluid.