When it comes to metal versus wood shields, wouldnt longevity play a role as well in their buying decisions? I mean, wooden shields from all I have seen tend to take damage much easier than metal shields. As has been stated, they may even be designed to do so in order to trap weapons in them. But metal shields are more durable. Harder to penetrate, break, or even deform, so while you might go through several wooden shields over the course of a campaign, a single metal shield would seem more likely to last you the entire time.
I know, we hate deadliest warriors, but thats what made this thought pop into my head. Samurai versus viking, the kanabo smashed the viking shield. Samurai versus spartan. He took a few really solid whacks, and barely dimpled the shield at all. It was still fully functional despite taking MASSIVE psi impacts. I guess to me it feels like making a choice between buying a cheap used car or buying a new car. The cheapo is cheap and easy to pay for sure, but its going to break down much faster and more often than a brand new car would. The new car costs way more, but its the only car you will likely need for the next decade or so.
Same for shields. That wood shield is going to break on you. Probably every time you go into battle, (Or maybe not, im not certain how fragile they are) But metal shields would be able to take far more punishment and last longer between replacements.