First of all, he dismisses the performance benefits, but frankly why am I even running the application if it's unused and in swap? Expunging it from memory with application-level semantics is a potential win because it can theoretically prevent you from ever hitting swap even after using hundreds of apps over the course of weeks or months. If you have an app with bad memory management this could be more than a trivial win.
As to the UX question, Apple is trying to push the envelope and say there is a better way to interact with a computer. Much like the auto-save stuff, Apple is re-examining quarter-century-old paradigms that geeks take for granted, but are perhaps not necessary in today's world of computers that are thousands of times more powerful. Now it may be that this they are overstepping and this is creating real usability problems, but it seems more likely to me that a good portion of computer geeks are simply uncomfortable with any change to assumptions they've had for the majority of their lifetime in computing.