Bront - Well, you are basically going to do 1 level of damage* less than a full Two Weapon Fighting character in exchange for 1 extra AC if you are using a heavy shield. If you are using a light shield, you will just lose 1 level of damage. This is because the Two Weapon Fighter can get Two Weapon Defence. However, the advantages of Improved Shield Bash over Two Weapon Defence is HUGE. You can get your AC even with only a shield, something TWD doesn't give you with just one weapon. So, if your primary weapon gets sundered, or lost or whatever, your AC won't change. Not so with TWD. Also, ISB effectively gives you another slot, as you can enchant the shield and the spikes SEPARATELY. So, you can have a +5 shield and a +5 weapon all in one package and you get to use both at the same time for one measely feat. Sure, you do less damage, but that is only 1 point on average.
As KarinsDad said, the sword and shield fighter is there to stay, not to blow. As an analogy, in Battletech, experienced players ALWAYS modify their 'Mechs to take full armour. Why? Because even if you do an additional 50% damage (more like 10-20% in most cases) from a few points you save on weight by skimping on armour, it doesn't compare with the 100% extra you get if you manage to stay alive for another round because of the heavier armour (more if you decide to Hell with the heat scale and do an Alpha Strike).
* Level of damage means from d8 to d6 and from d6 to d4, etc.
As KarinsDad said, the sword and shield fighter is there to stay, not to blow. As an analogy, in Battletech, experienced players ALWAYS modify their 'Mechs to take full armour. Why? Because even if you do an additional 50% damage (more like 10-20% in most cases) from a few points you save on weight by skimping on armour, it doesn't compare with the 100% extra you get if you manage to stay alive for another round because of the heavier armour (more if you decide to Hell with the heat scale and do an Alpha Strike).
* Level of damage means from d8 to d6 and from d6 to d4, etc.