Yeah that's the helm I was talking about, my mistake.
No matter what the stats for Defender are it's title says it's a +5. But either way it still grants the AC bonus. :D
But the item "near Beregost" is cursed. So technically you can get that high, but who'd want to?

Someone who felt that the loss of up to 18 hitpoints and a weakened Save vs. Death was worth the extra -1 to AC and better Saves against everything else. A good example would be a Cleric or Druid with 18 CON (possibly Dualed from a Fighter): Priests already have excellent Saves vs. Death, and the CON penalty would be meaningless to them, so the item would be almost completely without drawbacks for them.

ah but wouldn't a druid be the only one able to wear the sword, and unless it was a druid/fighter the armor would be unusable... so a druid fighter would get the greatest benefit from the AC lowering items (in which case the con penalty would be once again important, spoiling the whole thing)