The main problem with shields is that you usually don't find one until after lair, especially if you're talking about just shield or large shield (check the objstat results:
https://drive.google.com/folderview?id= ... sp=sharing). So there actually is a real xp cost, since you're not choosing between, say, 14 skill scimitar + shield vs 18 skill great sword. You're choosing between 18 skill scimitar + shield vs 18 skill great sword vs making your character worse until you do find a shield. I think the third option there is not very good, and it's not clear to me what is better between the other two options even if you ignore the xp cost of shields.
If you do find an early shield, then it's probably fine, but about half the time, maybe more, you do not. Bucklers are a bit more common but still rarer than the 2h weapons you'd prefer to be using in this part of the game (great sword, dire flail, halberd/glaive, battleaxe).
If your character has found good spells and is wearing light armour, then switching to using a shield lategame is spending 15 skill levels that could have been spell school levels, which is a pretty obvious cost. It's still a notable cost for plate characters if you have both good spells and wizardry, though that's not that common.
Lategame the problem with shields switches to them not working on what I find to be the most dangerous attacks, so I'd rather just kill things faster. Your mileage may vary, and if you find a great shield then that obviously makes shields better. If you found one early this is probably not enough for you to care about, but this makes me prefer 18 skill great sword to 18 skill scimitar + shield. Perhaps 18 skill demon blade + 0 skill shield is better, I have not done much 0-skill shield testing.
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Personally I always use shields if I pick fi, because why else am I picking fi instead of something else?