Interesting points.
At low armor, the damage reduction is close to 1:1, so you'll be saving ~6-7 damage * your shield skill damage reduction, usually around 4pts per hit. Kite shield has above average stats, but it's in the 16-18 armor tier where arbalests can shoot through them pretty easily. +10 armor would see a massive difference in durability, ~6 damage difference per hit. Going from 40 to 36 isn't a big deal, but usually it's something closer to 20 to 16, a 25% increase before factoring in health, assuming that damage to shields is similar to armor. -2 shield skill and the damage reduction is reduced by 12%. Going from 4 to 2, 34%->22%, close to another 20% durability loss. Both stack up relatively quickly.
Guess the gist is that +2shield and the better shield is close to a 50% durability increase (1.25 * 1.2). If you have higher shield skill, it's more durability in the realm of 2x% vs. weight.
Though I think there should be some incentive to use lower rank shields, I don't see the problem of making the higher rank shields the more desirable choice if you invest into shield.
I agree with this, but shield has a low difficulty threshold. I think many of the 5 difficulty shields are getting to ridiculous points in stats outside of the Triangle Shield which needs to be made 1 slot. And Fur Covered Shield is terrible.
Shockingly enough, some players want more weight on their shields to prevent block stun and crushthrough. The 4-6 weight section handles this just fine. I did ponder for a long time whether to change The Knightly Heater Shield's and Kite Shield's weight, but ended up just getting the former reduced from 5.5 to 5. I'm very hesitant to touch any widths except for ones that are blatantly out of place like the Green Crescent Heater or Long Pavise Shield. The way they worked were changed a few years ago and it's a bit confusing for me, especially coupled with the shield forcefield increase. The low width shields tend to just have better stats everywhere else.