Thinking fast is no crime against plankind. I would have to say yes. I would have to say some kind of composing and practicing process. I suspect not. Anything explicitly planned must have arisen, at some point in history, from a less explicitly planned activity.
Exploratory testing is exactly the kind of thing that comes before rigor and makes rigor possible.
ET is primarily creative. It has value in terms of the problems we find while doing it, but it also has value because it is a way to create other kinds of tests. It's a great way to learn about a product. And if I did want to make a rigorous test plan, I'd better start exploring the thing I intend to rigorously test. If you already know for sure what the next test is that would be worth executing, by all means follow that plan.