I think that the problem exists in both cases (if the rules don't say you can't grab a swarm, then you can; likewise, if the rules provide an exhaustive list of every possible difference based on the situation). Both focus on the rules instead of the "fiction". Now, it's not that the "fiction" has to be realistic or a perfect simulation of something - that's not what I think the problem is.
I think that's still the problem. You don't see a discussion here about whether or not Fireball should be able to damage a swarm because close to 100% of the people on this board don't see a problem with that. It also agrees with the rules. No one is suggesting changing the rules or allowing DM Fiat for Fireballs to allow them to fail to damage swarms. No one is suggesting that if your description of a Fireball isn't good enough then it shouldn't affect a swarm. No one is suggesting that anyone who allows a swarm to be affected by a fireball is too focused on the rules instead of the fiction.
However, change the word "Fireball" to "Grab" and suddenly all of those arguments have happened. And that's only because people feel that one fits the fiction and one doesn't. But the question is which fiction? I prefer to let the fiction flow out of the rules.
It's that, if you're concerned about what the rules say above everything else, you don't have as many opportunities to inject that "moment of judgement" into the game. The players don't contribute as much creativity to the game.
I'm not saying that the rules should be ignored, either. I think the rules should be followed to the letter (and ditched or changed if they don't work). What I think good rules do is that they give you lots of moments of judgement where players can inject that creativity into the game.
The thing is, the more "moments of judgment" you have, the more random, imbalanced and sometimes stupid the game gets. It's possible I just don't have as much faith in humanity, but nearly every time I allow players(or even DMs) to interject "moments of judgment", the more often the game degrades into arguments and weirdness.
As an example: "Well, the rules don't say whether a Fireball spell catches items on fire within its area of effect. But it would make sense within the fiction, so I'm going to say that everything in the area of effect is on fire. It is magical fire from the Plane of Fire, after all, not just any fire. Also, the rules don't say how fast fire spreads, but it's really hot and this place is made of wood, which burns really well. I'm going to say that your fireball weakened the building enough that it also collapses and falls on you. The rules don't say how much damage a two story building falling on you does, but I think in the fiction it doesn't make much sense for anyone to survive a 2 story building falling on them, so it kills you all."
Obviously, it's a bit of an extreme example. But it's pretty close to something a DM did to us once in a 2e game. I mean, it made "perfect sense" according to the "fiction" of his world(since that fiction was entirely in his own head), and according to "fiction' there just was absolutely no way for us to survive by doing something as "stupid" as using a Fireball spell in a wooden room(I got the impression he was annoyed that the fireball spell also took out all his enemies, so he got revenge....but he claims those weren't his motives).
Now, to me, if you removed all of those "moments of judgment" and replaced them with reasonable rules that were tested for balance and fun, I would have had a much better time.
I think that, in the third case, we'll see more personalized content from the people in the game.
But it comes with the disadvantage that each and every time he uses the power it's up to the judgment of the DM whether he can use it. What counts as a reasonable description of using the power? Only the DM in question can decide. The usefulness of the power then varies so greatly from table to table that there is no way to predict how good it'll be.
With rules like that, you'll play a Rogue in one game and you'll feel completely useless since Swarms, Oozes, Undead, Elementals, anything larger than medium size, any creature that you haven't seen before(how do you know where it's heart is, if you've never seen one before?), any creature with odd anatomy, Constructs, and probably a huge list of other things are immune to your Sneak Attacks. And besides, the DM says that monsters in combat are always on guard. They can effectively defend themselves from anything. The only time you can use the power is out of combat.
Meanwhile, someone elsewhere is starting a thread because they can't figure out how to balance the power of a Rogue with the rest of the group since their Sneak Attack is such a good power. After all, it works on 99% of all the creatures in the game(every monster has vitals of some sort), so they always get Sneak Attack. After all, when an enemy is paying attention to the Fighter who just hit them last round, they can't effectively protect their back and aren't watching for any ranged attacks. If they did, then they'd lower their guard against the fighter. So every single hit is a sneak attack in combat, which gives Rogues a significant damage boost over every other class in the game.
The problem is, "personalized content" tends to be only good as the person who comes up with it. And people vary in opinions SO much that the possibility of one DM ruling the same way as another is nearly non-existent.