The danger for me was thinking that I knew more than the professional did. Yes, you can be more knowledgeable than the trainer, but would you expect them to know more about, say Ruby, than you do if they dabble in it in addition to their full time job? Seems to me like you can (and probably should) always find a professional who knows her trade better than you do. Otherwise, yes it's a waste of your money, time, and health.
I would absolutely expect to find people in other fields who know specific areas of my field better than me. This is the story of every single person "in computers" since the origin of the career, everyone who dabbles desperate to show that they know some specific better than you.
