We increased the ACP on shields to balance them with Parry; it's not accidental. Small Shields have 3 ACP, Large 8 and Tower 13; you'll notice the increment of 5, which also is how much Parry you need with Shield Parry to get an extra point of AC. That means that they all have the exact same maximum optimized AC output (6 AC from Parry/Base shield AC, with the possibility of going up to 10 AC if enchanted or buffed).
A feat that would negate this ACP would essentially throw shields off balance. Your feat gives Tower Shields an extra 3 AC (since you negate 13 ACP), Large Shields an extra 2 AC and Small Shields an extra 1 AC. Basically if we implement that, we just essentially went back to NwN's original balance, except everyone, shield and non shield users got extra AC to boot. The point of this system is that all builds can be viable.
The reason Tower Shields are not efficient is their weight compared to the AC they give when using Parry. However, for non-Parry users, they remain the best deal around. The character with 10 Dex who won't invest in Parry is better served with a Tower Shield than any other option.