It's a roll. It's a roll like any other defense in the game. If you fail your shield roll, the shield does nothing. There isn't a "any strike they hit me with will almost always be blocked" because if you get hit, you failed your shield roll, and if you failed your shield roll, you don't block. If you pass your roll with dodge or parry, the same thing happens, unless there's some mechanic my Guardian is just utterly missing (which I would be very happy to be proven wrong about).Whenever I fight a shielded fighter, often times they defend against any strike I perform with literally no consequences to them. As of now, I could literally get any shield, go to the worst parts of the COGG universe, and be completely safe because any strike they hit me with will be automatically or almost always blocked by my shield.
You do get one bonus reroll with larger (not smaller) shields, as Gorth noted. Depending on your class, this may or may not be ideal - as Lexx pointed out, you're giving up a hand slot. Not only does this mean - if you're wielding a weapon - no non-occult lighting, but it also means no stealing, no main-gauche (bucklers don't give bonus reroll), no pulling up people who get knocked down (this is a big one for groups), or whatever else you need a free hand for. For (most) adventurers, Slippery Coward gives you the bonus reroll when dodging, as long as you're not attacking, so if you're a non-combatant, the Shield reroll's a wash for those classes.
Depending on your build, you also may just not have room for Shield Use - dodge replacer skills are an easy point here, but I want to say that I think (speculation) it doesn't work with Nightblade Tactics, either - Risk is noted as only increasing with dodges/parries. You also cannot use Tactical Dodge at all with Shield Use, which is a hit to several classes.
A bit incidental, but I believe Rogues can end up with the highest defense in the game, sitting in the 1106-1156 Dodge range (depending on if they want Slippery Coward or not) with the use of iquaj+Coward's Contingency, as opposed to the highest Shield Use cap being 975.
Of course, shields have the advantage of being the least affected by encumbrance, balance, position, etc., so for those who like to run around with 3-4+ encumbrance (plate armor, anyone?), it's likely going to be up there in what you should use, and obviously it's going to be the best defense possible for a Guardian, given their mechanics revolve around it.
This is noted as happening in the help combat file, but I don't think it is. Not sure if it's a bug or an outdated feature that was removed.1. Hammers and heavier weapons should cause at least some damage even when holding a shield because of the amount of force you're applying to hit that shield. Most combatants in real life would at the very least suffer some type of minor damage when being struck with a hammer even if they have a shield on their arm because of the force of these weapons.
edit: Something I forgot to mention: Rias has noted that guards (not Guardians) are eventually not going to be able to defend other players with dodge, so Shield Use is going to be the selection of choice for escorts in the future.
In a vacuum, with nothing but Dodge vs Parry vs Shield Block - yeah, the one reroll Shield Block gives is going to be an advantage, but there's a lot more to consider than that.
