I always thought shields were more so for people who didnt want to invest in Parry, rather they be a priest who I dont think has the points to spare (or might not even be a class feature, never played priest), or other classes who didnt have the points to invest in Parry. Shield parry even now I thought was made to make bucklers/large shields more viable, since tower shields are fairly taxing on skill point penalties.
But, as a two hander boi, I think it makes sense as shields are much more for giving to people who dont know HOW to block a sword with their own. And, while one wouldn't hate to have one in duels, it makes sense that an individual (meaning if you come from a background where you went out on your own/ weren't in a tight unit) you might opt for lighter, less cumbersome ways of attack, hence the two hander exists. And while perhaps late game people without shields may rise higher (which I defer because I have never gotten a shieldbro up that far), a lot of people dont get to that far in the game, and besides that it means you can invest in other skills that parry boys cant.
And to be honest, if two handers/ no shields were completely nonviable as a defensive option, no one would ever had used two handers historically at all, which to me means that you should look at it in the perspective of what skill points can I invest in besides parry to punish those that have to put in hella parry points.
(Like antagonize, for example)