I work at the admissions office (actually posting this from the front desk right now, what up) -- the interviews have absolutely zero weight. They're allocated on a first come first serve basis, I believe -- and we've been repeatedly told by officers that the interviews are a formality, and nobody should worry if they don't get one. About half of all current students (myself included) were never interviewed. International students in particular will not be interviewed because there just aren't enough alumni out there, although if there are special circumstances obviously the office will make an effort to arrange an interview.
Long story short,
1. They definitely don't pick people to be interviewed based on merit -- in the US it's the luck of the draw, and outside the US, it's if there's something really unique which suggests an interview is necessary
2. If you don't get interviewed, you are at no disadvantage