As someone who uses Banners a lot, especially for adding waypoints to Maps, it is somewhat discouraging when my waypoints vanish from my Maps because the Banners I placed got destroyed by decaying due to being placed in Ruins, or just because of natural block decay in the wilds. I think Banners should be an exception and not decay, at least just in the wilds, but I really think it should apply to Ruins too. This would just be really convenient so I have an easy way in-game to visualize where I have been and what is where without needing to write down coordinates in a book or use an external software to draw a map.