I personally wouldn't mind if this buff were to be implemented into the game, but I don't think it makes much sense.
There are a limited amount of permanent spaces, creature spaces, and 1 each for items. When you "summon" two Shards of Freedom, they don't take up 2 permanent spaces, because SoF stacks. However, there are a limited amount of non stack-able permanents that you are able to play. After a certain point, you will simply not be able to play anymore permanents. The rule for shields is that you only have 1 space available, but when you "summon" a new shield, it will automatically dismiss the former shield instead of giving you a message like: "There are no more shield slots available". So, even if Bone Wall is a shield that has the potential to stack further when creatures die, the rule still remains that when you successfully use a shield card, your shield slot is completely emptied. If this wasn't a rule, you'd have to use some form of PC on yourself in order to switch shields voluntarily.
Bone Wall is a stack-able permanent, but it is also shield, so other rules apply as well.
