As far as I can tell the black inset in the phone is "cut" with a downward slice (think of it like a cookie cutter coming down over a sheet of aluminum) which is then right justified when the phone is assembled.
Since the edges will always crimp down there will always be light leakage on the side of the phone it's not set against which clearly is the left side.
No lie there are 20 people at my job who got the new iphone, 100% myself included, have some light leak on the left. In fact, I've yet to meet anyone with none. Clearly this is done so as to make sure there's less material and it always fits as opposed to having too much and requiring a manual shave to perfectly fit on each unit.
Honestly, it doesn't really bother me. Dead pixels would drive me nuts, but light leak seems not to even catch my eye.
Point is though, if you have no dead pixels and your battery is doing decently, I'd keep the phone. You have an unlikely chance of getting a phone with no light leek so why risk a crappy battery or dead pixel on the screen?