Hi,
Fact is (as your faq states) that calories in has to be less than calories out for someone to lose weight.
So what's the benefit of having a protein heavy day one day and carb another?
As far as I'm aware, there used to only be one SW plan, which is what we now know as 'Red Days'. This was based on having protein as Free Food as it was thought to have a good satiety level, and allowing small amounts of carbs.
'Green Days' were introduced in the late 80s as a way of allowing vegetarians to follow the plan, I think. In this case, carbs were used as the Free Foods and small amounts of protein included.
The 'science' behind this is based on the volumetrics system AFAIK, which has to do with this thing called satiety - how filling a food is for its calorie value. Ie, a plate of pasta might have the same calories as a 100g bar of chocolate, but it will fill you up a lot more and hence you won't want to eat anything else for a while. The chocolate is not going to stop you feeling hungry.
As you say, it all comes down to calories in versus calories out, and this is where satiety is very important. When I first started SW, I added up the calorie total of my day's food on several occasions and was amazed to find it coming out at under 1500 calories. I felt absolutely full and would not have believed I'd consumed less than 1500 with what I'd had to eat.
Personally I think the psychological aspect of SW is hugely important. It's been shown over and over again that if you tell someone 'you can't eat that', they immediately want to eat loads of it. The reverse seems to be true as well: if you say to someone 'you can eat as much of that as you like - have as much as you need to fill you up', they tend to stop once they're full. They don't have that compulsive need to eat as much of it as possible.