One time I had an anomalous cook with high temps that wouldn't come down even with all vents closed. I figured I was drawing air through a seam. I make sure there are no visible gaps by spinning the sections until they fit and never had a problem since.
I don't think I've seen smoke leak from the lower seam, I assume air would be drawing in there.
IMO you want some air flow to keep proper combustion and to dry the bark. I think offsets need to be air tight because they are tough to keep up to temp.