Shields are similar to cav due to the fact that you get a double benefit from raising shield skill and gaining access to objectively better shields. 0-2 (and 5) skill shields were buffed, but they still have crappy durability with only 0-2 shield skill. If you have 4-5+ shield skill, you wouldn't have any incentive to use the lower tier shields outside of the slightly lower weight anyways. That's why I think that the stats don't need to be very far apart. The heavier round shields haven't changed their stats in a while, and Round->Heavy Round->Huscarls are roughly +3 armor and +50 hp at each step. The Elite Cavalry Shield has below average durability in its class because of its prowess in all areas, so the light round shields are all pretty close to each other with only minor durability increases.
I borrowed a +3 Round Cavalry Shield from the armoury and it still breaks very quickly in-game with 2 shield skill. I need to have my shield sheathed a lot of the time.
I agree that there should be some incentive to use the lower rank shields over the higher rank shields, but I feel that the weight differences are too much for certain shields. Compare the Heavy Kite Shield to the Kite Shield. For upgrading to the 1 difficulty higher requirement Heavy Kite Shield, you get +7 armor and lose 15 hit points at the cost of 1.5 weight. I understand that the +armor makes it more durable despite the hp reduction but is that really worth 1.5 weight when you also put another point in shield just to get access to it? Or look at the Horseman's Heater Shield vs. the Knightly Heater Shield. A Knightly Heater Shield, which is 2 difficulty higher, gets +10 armor and +12 hp at the cost of 1.5 weight. Of course, the +10 armor will make it a lot more durable, but I'm not sure its worth 1.5 weight considering it also takes an investment of 2 more shield skill which could go towards something like IF.
I don't think the durability should be increased on higher rank shields, but I think the differences between the weights should be slightly lowered or the shield speeds/radii should be slightly increased. Even if you compare the shield situation to horses, compare the difficulty 4 and 5 horses to the unarmored 6 and 7 horses. There is pretty much 0 reason to use those lower difficulty horses if you have 6 or 7 riding skill. 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.