All that just begs the question; why would anyone do this, ever?
If you attack with a shield, you lose your AC from that shield (unless you have the feat). A shield does rather poor damage (1D3 for light, 1D4 for heavy). A spiked shield increases it to 1D4 for light, 1D6 for heavy).
Or, you can just attack with spiked armor, always do 1D6 and not need the feat to keep your shield AC. Also, spiked armor is a light weapon, so you can take less of a penalty.
I suppose there might be times when you really, really, need to do bludgeoning damage, but I don't think that's enough of a reason for anyone to plan on shield bashing a lot. After all, there are times when you really, really, need to do piercing damage (such as from armor spikes) too.
It seems to me that the ruling (I think in the FAQ??) that you can use two-weapon fighting with (anything) and armor spikes pretty much killed the shield bash.
One thing I really dislike about D&D right now is that there are a lot of clear winners when it comes to equipment. A great sword does more damage than anything else, has an equal critical rate, and is easily affordable. So if you use anything but a great sword, you are a step behind. In my game, I have changed them to 1D12 damage to make the other weapons at least compete.
Some things are better in all but one respect, which is ok to me. For example, full plate is better in every way than half plate. But, the extra cost makes up for it a little in my mind, so I have left it alone.
But now armor spike two weapon fighting is better in every way than the shield bash. Here we go again…
Is there something I'm missing? Is there at least one redeeming feature of the shield bash?
-Tatsu