Every now and then I will grab a 0 slot weapon and a buckler so i can carry two siege shields. They have a variety of uses. On open plains maps you get the cover that normally requires a shield wall of at least 5 players. You can turn corridors into impassable slits for archers and crossbowmen to fire down.
The thing I like about them is that they are the best way to get your team to do something other than charge every single round. No matter how much you type in teamchat, no matter how many voice commands you make, no one will follow. You plant two siege shields in a good strategic location, and it's like watching a coral reef of players form. First the ranged characters will set up camp behind them. Then a few shielders and long polearm users will show up. Soon you have a thriving ecosystem of players, a beautiful cluster of strategy in a sea of ignorance, leeching, and WWWWWWWWWWWWWWWWWWWW.
So, the siege shield is working. It's doing its job. Why should you be able to make it better? Because beyond arrows and bolts, the siege shield falls apart like wet paper. Most 2H and some polearms can destroy it in a single hit. A +0 Dagger can knock it down with two thrusts.
I think the item is great, and has much less griefing potential than ladders. You don't need to spend time typing or navigating voice commands. You don't have to set up batallion flags. You just drop those siege shields and it's all good.
If this idea does get seriously considered, I'd suggest making the shields a little stronger but also reducing their weight as they receive heirloom levels. Even in really light armor with a dagger and buckler two siege shields (50 weight) slows you down a LOT.