The purpose of showcasing Shaman in a PvE context is to act as a control for the next two comparisons, as monsters in aqw do not have primary stats and thus, do not experience the effects of INT shield. This can be deduced very simply by observing that your magical-based attacks will deal the same amount of damage regardless of the enemy in question. Here the auto does 159 non crit, 429 crit and the first skill does 689 non crit, 1865 crit.
In this next example against EPL, you'll notice that the non crit and crit values from Shaman's auto attack remain unchanged, due to being physical, whereas the 1st skill's values do change since it is a magical attack. At a total INT of 241, with no other damage resisting modifiers, the non crit value of Shaman's 1st skill is reduced to 466, and the crit value reduced to 1260, which results in a total damage negation of ~32%.
In this final example against Necro you will again see that the auto attack's damage output remains unchanged, and in this case, the magical damage output of the 1st skill has dramatically decreased. What is interesting to point out here is that Necro is only sitting at 410 INT in this example,what's even more interesting is that increasing the INT by over 92 had no further effect on the damage negation, still it remained at 80% negation. But why is that exactly?
One thing I neglected to mention up until this point, is that Necromancer has a r10 passive which increases its resistance to magical-based attacks by 30%, and I assumed that this would interact multiplicatively with INT shield's effect. However, since increasing INT above the point where it would supply >50% damage suppression had no further effect on its overall damage negation, it could only be deduced that INT Shield and Mag In interact additively with one another. Not only that, but they both share a common cap, at 80% damage reduction against magical-based attacks. Of course this is very intuitive, seeing as they both concentrate on magical damage specifically, however it is still an important realization to come to. In conclusion, INT Shield, at its core, is essentially a Mag In that scales with your INT.
Perhaps the biggest, and most obvious limitation in regards to INT shield is the scarce applicability, really only being useful in certain PvP situations, and with so few players even having an awareness of INT shield, its presence in the meta is not so easily felt. It would be interesting, in future, if it were possible to put a system in place where monsters could not only dish out magical damage, but could also tank it as well, lending to a more varied, and wider gameplay experience, where certain bosses, for example, could call for a caster-heavy group of players due to an inherit weakness to magic or, conversely, certain bosses with high resistance to spell attacks would require a more Warrior-heavy group of players. Having this level of variation would be good for gameplay, as it would require more thought/careful planning, as well as some degree of communication with your fellow players on what the best approach to take would be.
As for uses well, it does to a certain extent fulfill the role it was intended to play in giving physical classes just that little bit of resistance against magical attacks which is needed, especially considering most physical based classes tend to have low-medium ranged attacks, whereas casters are more likely to have long ranged ones. There are also other factors at play that result in caster classes being inherently more powerful than physical classes, more on that can be seen here in the 'Primary/Secondary stat conversions' section. At level 90, with full luck enhancements, a tank-melee class would have a total of 91 INT, and would yield a mag in of ~12%, which sounds low but is actually pretty significant, especially if said class had other forms of damage mitigation alongside this. Also if anything, this gives yet another reason why full luck is more beneficial for physical classes, as the additional INT derived from your luck stat drastically improves your total INT value, compared with fighter enhancements, in which you would only have the base INT.