Well, I've had horrible experiences with patches (auto-adhesive ones, mind you) in the past therefore I always replace my tubes even though it ain't cheap (3 euros per tube). Still, I usually end up using up the old ones for something or other, whether it be for rubber bands or for a chainstay protector. I might give the proper ones (i.e. non auto-adhesive) a shot sometime in the future but I'll always carry a spare tube around anyway.