Guys, this is DCP. You don't want to tell him to stop playing alone and start gambling. Then his practice is just gonna cost him ugly amounts of money, and make him feel even more negative than usual if things don't go his way. He gets stressed and agonized enough when he gets a so-called bad roll in a drill. Imagine him getting an actual honest-to-god bad roll that costs him 100 bucks.
I think some players reach a level where maybe they're a shortstop and they want to hit touring pro level or something... those guys may need to get out the wallet and start putting themselves into tournaments and gambling situations.
About 'practicing and playing aren't the same'... yeah, playing by yourself is a form of practice going by any dictionary definition. I think when an instructor says it isn't... he isn't looking to make a point about the definition of a word, he's making a point about what's effective practice and what's a waste of time.
I'm with the instructors on this one, playing racks by yourself is not very effective. If someone is willing to settle for the results of this form of practice, they're selling themselves short. Not every sunk ball or successful position improves you. You can run out a rack of 8 ball and not have learned a single thing that you didn't already know. Worse, you can do it with a lot of ineffective habits and bad thinking and you won't even be aware of what you've done wrong.
