I don't think you understand what he means. Unless you play to 99%-100% of your classes potential (You don't, unless you're in a top 5-10 world guild anyway generally) that 5% dps difference isn't going to show. Once you have EVERYTHING about your class/spec down to a science (Perfect CD useage, perfect rotation with no mess-ups) THEN you worry about the math that says "This spec is 5% behind".
The difference is noticeable way before that level of play. Think of it like running track. Sure, if you're a high school runner, you might not notice the difference if you have custom-fit sprint shoes that shave 0.1 ounces off the weight and fit your feet like a glove. If you're a professional, that little edge can be the difference. But even an amateur will notice the difference between racing spikes and trainers, and will definitely notice the difference between any sort of running shoe and your dress shoes or snow boots. So why go raiding in your loafers instead of strapping on some appropriate footwear.
Honestly, I feel like this is just an excuse for people who underperform. It does matter, and it makes a difference, and you're wasting the time of a bunch of other folks if you're not doing what's reasonably in your power to help pull your weight in raid encounters. Spend hours theorycrafting, and thousands of gold reforging and regemming for a 1% increase? Maybe not. But click on your second spec to get 5-10% damage? Yes.