Essentially, it depends on who you ask.

The main idea behind it, for the most part, is that it helps you keep your eating, and thus calories, under control.

Many who once stood behind the concept of seperating are now no longer strongly advocating it.

It works, but mainly because of calories, not because of the seperation from what I gather. This is just a large general statement though, so take it as that.

Other than that, it really doesnt make a large amount of difference. I seperate myself, but its just because I am so used to it that its easier to pack for the day, etc, thus keeping me working towards my goals.