Ah the old " AC is/is AC useless" chestnut. These discussions have often turned heated in the past, largely because people are talking about different things. Now most people have largely given up on AC though, because of the major developer fail that is Epics. Here's a rundown of the "warring factions":
1. The "Hit only on a 20"-crowd, also known as the "AC-nobles". This group consists mainly of intimi-tanks (but also houses some ridiculously well geared/built DPS builds). They have two criteria; first they demand that foes only hit them 5% of the time, i.e. when the mobs roll a 20, and second they only care about raid bosses. If that benchmark is not reached, AC is quickly dubbed "pointless". All or nothing.
The intimitanks among them don't care much about trash mobs - they seldom solo since they usually have low to middling DPS, and bring brawny toons along in their entourage to sort out the "riff-raff" anyhow.
Many are a bit lost now since intimi-tanks have gotten a big fat middle finger from Epics, and some have been put into dry-dock. Or they become TR:d - with their old AC gear hanging oiled and polished in their TR-banks waiting for some new tomorrow.
2. The versatile melee-builds. This is people that like to play self-reliant characters, which includes healing, UMD and OK defense. They often have an AC around 60-80, they easily solo when nothing else is up, and often see "miss" messages in Amrath quests on Normal diff. They don't fully recognize the AC situation described by other factions, since they don't have a "boss 5% to hit" criteria, and only see AC as a complement to their characters along with UMD and self-reliance. These builds don't fare as well in Epics as factions 3 and 4, since their investments into AC gives them absolutely nothing back there.
3. The fallen from grace ex-nobles. This is people that used to belong to group 1, but have given up. Maybe because they couldn't grind enough gear to belong to the AC-Nobility, or their 5%-hit zealotry just pushed them over the brink when new content meant they couldn't keep it as tight. They play their old toons now re-configured for DPS (or have turned to casters in their disgust). Many still have an OK AC but refuse to see any "Miss"-messages in their combat logs that doesn't come from Displacement.
They are very bitter about the whole affair - even though they would never admit it - and their conviction that "AC is worthless" is almost religious. Epic-difficulty favours these kind of characters since they are geared out and don't care so much about defense.
4. The no-AC toughs. Here we find the various knuckle-walkers who would have a hard time reaching a meaningful AC even if they wanted, and thus don't care. They need two healers strapped to their backs with the blue bars intravenously attached to their red just to get to the quest anyway, which leads to the conclusion that "defense is not their problem". Which is true for Epics.
But they often pop into threads about AC mumbling "ejsee iz wuthless"; because if that would be the case, that could mean they're extra uber since they commonly have an AC around 8. Fortunately the game designers have included copious amounts of DR and HP in their loadouts instead.