Inter_211 or RuneEdge, you can't argue both ways. Basically Wingback is something in between a side back which is a defender and a winger which is a midfielder so you can't say it's really a midfield OR defensive position or its dumb if its either one but for the sake of being able to select it, it's just like Inter said, a midfield position
You're all looking at this in the wrong way. A Wing Back is exactly the same thing as a Side Back. But in the game, they are two different positions so that you can control how aggressively they attack and track back.
In reality is just a position where defenders block the wings. Roberto Carlos is a wing back. But he's not a midfielder.