I'm pretty sure it is just cosmetic. I'd just check to make sure the barrels shoot as well and regulate without it, and if they do I would not put it back. I have a similar sort of double rifle, and the gap between the barrels doesn't bother me a bit. It seems to shoot and regulate well without it too.
But if it looks better to you with it in - enough to make up for the trouble of taking it out and putting it back in - then have at it. I definitely would be wary of rust under it though, it is almost ideal for that, if the rifle's used in damp conditions.
