To some extent, I have done this at times and still incorporate it at times. In fact, it has only been over the past month or so that I began at least loosely planning my workouts. Even then, the workouts are only loosely planned. It was only as I was getting dressed for my run today that I decided to do 2:00 repeats today instead of the 1:00 repeats I had been doing some of recently. I decided I needed to push the length to work a little more on stamina. As a race nears, I usually have some plan because I want to make sure I cover all my bases. However, when I don't have any race on the schedule or I only have a very unimportant race on the schedule, I don't plan, I just run. On weekdays, if I feel good, I throw in a workout. If I don't, it's an easy day. On the weekend, if I feel good, I go long. If I don't, it's an easy day.
Personally, I think what you're calling no-plan training is great for base training. As a race nears, I think there are holes in it. Personally, if I didn't plan, I would probably skimp on the kinds of workouts I would most need. I don't like them so, if I don't make a conscious effort to get them in the plan, I might not do as many as I need to be at the top of my game.
Interesting topic, though. I wonder who else has tried this type of approach and what kind of success they may have had.