So DJ bought a Trackman to work on his wedges instead of getting the last 1-2% out of his Driver.
Everyone will derive benefit from improvement of the long game but we ought not lose sight of the fact that improvement in the long game is much harder to achieve than improvement in a lot of other aspects of the game. For example almost everyone can become very competent putting inside five feet, but almost no one can drive the ball like Dustin Johnson. Therefore it would be wise to devote your practice time to areas where you are most likely to see results.
What I think when I read "It's better to hit 9I from the rough than 5I from the fairway" is "how the hell am I going to get another 40yds out of my Driver?" I have a full-time day (sometimes day and night) job, a wife and two daughters. I don't have the time to work my a** off in the gym for those 40 yds. But I can learn to pitch and chip close to save par from off the green where I hit my 5I.
This was the quote that I had the most issue with. It totally depends on the type of grass and condition of the rough. Sure, on most munis, this is absolutely true. But I have played tons of courses where the rough was VERY penal and the 5i from the fairway was far easier than the 9i from the rough. So all this is dependent on the course conditions. I played a course last week where you had total death outside the first cut in the fairway and all around the greens. Most stressful golf I ever played. You literally were dropping a ball if you were outside the first cut. The fairways and rough were paspalum which is tough as h*ll and hard to hit through. Even shots from the first cut were iffy. The rough around the greens heavily affected my short game. Didn't succeed in a single up and down through the entire round. So it's best to know your course before playing for a serious score and altering your strategy accordingly.
On tour where the rough is much more penal than your average muni and more penal that what almost all amateurs regularly play the proximity to the hole is significantly closer with 9 iron from the rough than a 5 iron from the fairway. And they are much better long iron players than your average golfer. I don't think most ams realize how few greens they hit with a 5 iron in their hand and how far from the hole they actually end up. The GIR % will be considerably higher with a 9 iron even in thick rough than a 5 iron from the fairway.