I count "sugar burner" as an injured metabolism. "If only 100% works, in the beginning then why bother?" Because the hard work now will pay off later? Because if you're only 80%, then your body will still have enough sugar that it will stay a sugar burner. Your body won't be deprived of sugar enough to understand that it's supposed to shift to fat-burning beast. It's like climbing a staircase. You can't just step up half a stair half-assed; you need to step up the whole way. For newbies who are heavy sugar burners, I guess it's possible to ease into the 100%, but at some point they have to get those carbs and bad oils down, at least until they go through carb flu.
That said, yeah, I guess a newbie could become a fat burner on diligent 80/20, or a relaxed 90/10, or whatever nuance. But it takes longer and it could get dicey. Look at all the threads on the forum started by newbies who say they went "primal" but they aren't losing weight. Only to find out: they are eating sugary greek yogurt. They are eating too much rice. They think 80/20 means two cheat pizzas a week. They are drinking too much dairy. They still like fat-free chicken breast. It never occurred to them that beer wasn't primal. They are eating too much chocolate. Their PUFA's are high from too many of the wrong nuts. And so on. In other words, they are eating a relaxed 80% too soon, and they are dancing too near to SAD end of the spectrum. The first thing I ask them is if they went through carb flu. If they haven't, that's a sure sign that their metabolism is simply not switching. That's why, with newbies, I err on the side of being strict.
5'0" female, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.