I'm not completely convinced of the usefulness of the Shield. It gives less dice than either the sword or spear, and when using the spear at least, you can certainly be better defensively than a shield. 
I'd be tempted to house rule this to give either a d10 die which can only be put in the left hand, or 2d8 which both most go in the left hand. This ensures it's the strongest defence, and doesn't allow you to have a better offense than the other options. I'm leaning towards the d10.
(Although Shield is an _excellent_ trait to use for Creative Abilities. Very easy to rationalise into play.)

Am I missing something?

Based on your wording, I wonder if you are treating the weapon dice properly.

The spear is [d6 d8]. One of these dice goes in your left hand. One goes in the right. They cannot both go in the same hand.

The shield is d8. It can ONLY go in the left.

So, a spear alone can never have "more defense" than a shield alone.

The shield die is tuned to slightly favor offense as a general tactic in battles, which is what I want. The real defensive monster is the spear/shield combo, with 2d8 in the left hand and d6 in the right.

Looks like I was doing something wrong - I wasn't aware you could use them both at the same time.
If you do that, do you get the Ability die for each skill as well?

No. Check out pages 48 and 49. You only ever roll one ability die (before adding creative abilities, of course).

The spear description (p. 49) specifies that you can use a spear and shield together. The sidebar on page 51 gives a list of all the possible weapon combos.

Now that you mention it, I remember reading that about the spear. Oops.

Now that I'm familiar with the way weapons are supposed to work, there's a question I have about the text.
Under Spear, it says, "You may use a spear and shield at the same time."
It doesn't say this under sword. But in the weapon combinations list, it clearly lists Shield and Sword (as well as Shield and Javelin, etc.).
So why is that comment there under Spear? Is there still something I'm missing?

Also under Sword, it says: "One of your sword dice may be moved to your other hand." Does this remain true if you have something - like a shield - in the other hand? I'm assuming so, but just want to be sure.

Spears are two-handed, which is why you get a spear die in both hands. The text clarifies that you can still use a shield even though both hands are busy. A sword doesn't require both hands, so sword and shield is sort of a given.

Sword dice can be moved back and forth whether you have a shield in your left hand or a sword. If you have a sword in your left hand, you can also move a die over to your right for serious smackdown.

Matt has it right.

Sorry for the weapon confusion, Darren. That was one of the trickiest parts to explain in the text, and I think it could be better. But I think you're on the right track now.

Yes, that clears it up. Thanks.