There was a stray comment in a blogpost a few days ago about revealing the armor class of monsters to players during a fight. My initial reaction was to think that I wouldn't normally do this, but then I started to reconsider, in light of the way I think about armor class.
Originally, armor classes were "hard" categories: each type of armor, such as chain, had a specific number which represented that armor type. You used that armor class to look up the number needed to hit an opponent. You then added any defensive bonus to that target number, such as the pluses on magical armor. You did not modify the actual armor class at all.
This changed pretty quickly, and people starting talking about opponents with an AC of 5 wearing Leather Armor +2. But this change is not an entirely good one, since it now introduced confusion: why is it called "Leather Armor +2" if you subtract the 2 from the base armor class? Why isn't there an AC 1 or AC 0 on the table? What do you do with negative ACs? It's no wonder that people started house-ruling ascending armor class as a replacement, and it eventually became standard.
But aside from the fact that I just prefer the old way (I've got the original ACs pretty well memorized, after 30-some years) and the fact that descending AC works great with a Target 20 approach, there's a very real problem connected to both the open-ended ascending AC system and the modified descending AC system: do you reveal the AC to the players? If you are letting players roll the dice themselves, telling them either the AC or an AC-related target number reveals information about the opponent's defenses. On the other hand, you'd expect characters to at least have a rough guess as to how tough to hit an opponent looks.
The "hard" AC system solves this. If it looks like chain, it's AC 5. If it looks like plate, it's AC 3. You can tell the players this information and keep the modifiers and target number a secret. Using the Target 20 approach, tell the players to roll the die, add their level (half level, for M-Us, or 2/3rds level, for Clerics,) and add the AC, then tell you the result. You compare this to 20 + magical bonuses or other modifiers. The players will not know exactly how tough a monster is; if they score what they think should be a hit, but you say it's a miss, they will know that it has a defensive bonus; with continued attacks, may be able to guess exactly what this bonus is.
It's the best of both worlds.