For the purposes of discussion (and someone please correct me if this got modified without my noticing), the chances of skull shield triggering are (0.5 / HP), so:
50% - 1 HP monster
25% - 2 HP monster
16.67% - 3 HP monster, etc.
That's per attack (naturally) so that's a lot more than two triggers per game against fire rushes, graboid rushes, and other high-att-low-hp things.
You don't know how much the RNG hates
Everyone, do you? It took me forever just to confirm that it wasn't the buckler that made upped skeles, and that it was the upped creatures.
Against Fire rushes, true. That is probably higher than two, probably more around three or four.
Against golem rushes, the HP raises constantly, so the RNG hates you more. I guarentee you will (Almost) never see a summon sick golem die from the effect.
Against Shrieker rushes, that is a 12.5 chance on the shriekers. You'd think it'd happen more. Nevertheless, the equation is right, more or less. Also remember that High-ATK, Low HP also means they kill you quickly, giving it less chance to activate.
Is a hard counter to forest scorp rushing. for the same reason as
half the shields in the gameHas about the same likelyhood as a fire rush should you encounter a munchkin/frogtosis rush.
Phase Dragon rushes suffer very little to this shield.
I could name dozens of other decks of how you describe, where the shield activates on average,
exactly as many times as I said.