Comparing Full Plate +2 to Adamantine Splint Armour is a false comparison. You can get Adamantine Splint Armour at the end of Act I - around level 5-7 range, but Full Plate +2 isnt available until level 11. So for Act II Adamantine Splint Armour is better than Full Plate +2 .. because you can't get Full Plate +2.
If you compare it to Armour you get around the same level - say Armour of Persistence, or Helldusk Armour - it comes out as slightly worse.
It's also worth noting that +1 AC reduces your chance to get hit by 1 in every 20, where as the special effects on most armour generally allow you to reduce enemies effectiveness or to kill them quicker - Reeling on the Adamantine Splint Armour effectively gives your party +1 AC against that enemy for 2 turns. And as we all know, a dead enemy does not make Attack Rolls, so killing an enemy 1 turn faster means they party takes less damage than you would do with that enemy alive and your +1 AC.
Let's look at Barbarian.
Potentially, AC 20 with Dex 20 and Con 20.
You might get con 20 from the Barbarian garb that boosts AC and a natural of 18. But you just don't have the points for dex 20. So you're stuck with dex 18 with gloves of dex. And there's only one of those. That's AC 19.
Also AC 19: Half plate+2 and dex 14. The same dex 14 your cleric and druid probably have.
You can get an amulet that set's Con to 23 so that would also give you AC: 20 with the dex gloves
