Shifting away from the endless "utility powers don't provoke" argument attempts (I'm fully in the "all ranged and area powers provoke" camp, btw, though I understand the other point of view) - if the paladin is up against one opponent, he can use this just fine, because you can't take opportunity attacks on your turn, which applies to monsters as well. If he's in a crowd though, it's a bad idea to try and use it.
Clarifying examples:
Party is fighting big bad solo monster, solo monster crits Pally or Pally's ally - Paladin uses Armor of Bahamut during solo's turn, solo can't take OA on its own turn, Paladin is happy.
Party is fighting a mixed group of monsters, Paladin is in the middle of the enemies, enemy A crits Paladin or Paladin's ally - bad idea to use AoB here in most cases, as all of the other adjacent enemies would get to OA the Paladin (though there would be some specific cases where it would be worth the risk).
Party is fighting swarm of minions - a minion crits, Paladin doesn't bother to use AoB, since minion crits don't do extra dmg, Paladin is unhappy due to not being able to use his feat.
So, generally, it's pretty rare that it'll be useful (kinda like most of the channel divinity feats).