I keep it, for some reason I have just presumed that it was there to keep the butter FRESH. Not sure how I came to that conclusion as, thinking about it, towards the end of the tub it does tend to start look rather unappealing. Maybe, having seen the overwhelming consensus on here, I will chuck it at the first oppurtunity and see if it improves my butter eating experience.
I've taken the liberty of presuming that spreadable butter blankets are also allowed, as I don't buy marge.
I normally keep mine - force of habit inherited from my mum, who never throws butter foil away in case she needed it for greasing cake tins.
Given that she never bakes cakes either, you've now got me thinking that she must have a stash of about 1000 butter foils somewhere in the house. I'll have a root around next time I go to see the folks and get back to you.