I have to both agree and disagree with this statement.
I think at the very beginning, when people start working out, how hard the workout is for them is important, as long as it's achievable. The feeling of having pushed yourself through a workout, and out of your comfort zone, is one of pride and achievement. It shows you that you are capable of becoming more than you thought you could be through hard work. This build of discipline and mental toughness/grit is a pretty handy thing to have when it comes to achieving goals.
On the other hand this usually builds up an attitude that the harder you work the better the results, which isn't true. You workouts needs to measured by results and not a feeling of how hard it was. That's not to say workouts are easy, but in the end there's a better way and worse way to do things. For example you could squat a lot of weight incorrectly and put yourself through back pains and knee injuries which would definitely feel harder than doing it correctly, but the correct form gets results. Or say you're aim is to lose fat and build muscle, so you decide to jog 8 miles every day and do lots of body weight workouts (which just sounds hard typing it) compared to doing weights some days and cardio on others (both short session and a lot easier); the second will get you results faster, easier and in a healthier way than the first option.
I guess its a matter of hard work vs smart work. Smart work will always pay best, but that doesn't mean hard work won't be involved. There's a reason personal trainers are qualified and range in specialties.
Hope my answer's done some justice to your question. Vote it (or whatever it is) if you like it. It's some good motivation to keep me answering random people on the internet :),
Note: I am not a qualified trainer so the above workout routines are only examples and not professional advice.