This advice is super generic.
You can only really create a complete "fleshed out" design upfront if you're walking a very well trodden path.
Playtesting isn't useless. It's how fun games are made.
Perhaps people are disagreeing with this because they haven't tried to make a game. A curious phenomenon.
"Listen to your users" is not advice, it's the end goal. What's the most effective way to listen to your users? Professional playtesting? Open alpha/beta? Forums, IRC channel, a subreddit? Frequently scheduled AMA with the devs? Infrequently scheduled AMA with the devs?
If you want to watch someone doing this, pull up some videos of Notch making games. That's the process he follows constantly, from start to finish. He also has good taste, which is why his games turn out well.
If you don't have a nose for what's fun and what isn't, you have to take feedback from your users. If it's a multiplayer game, you should take feedback from the most competitive users, ideally from the people who are trying to form pro tournaments around your game. Casual players don't care what you do to the game, because it doesn't affect them much. But competitive players are your lifeblood, even if you don't know it yet. A competitive scene is what carries a game after its shelf-life has expired.
Playtesting is key but its far from being a simple step in itself and it never tells you exactly what is the right solution to fix things. Thats why making games is hard.