Most advanced armourer useful, by faction:

[Maklodes]

I'm not sure if anyone will find this useful, but I decided to look over the possible armor upgrades and find out which armourers are useful to whom.

Typically, the most advanced armourer a faction can use is the Master Armourer. Often, there are very few units that can use a Master Armourer (example: the only Russian unit eligible for a Master Armourer upgrade is Tsar's Guard). In most cases, their most advanced units start with partial plate, and upgrade to full plate. In some cases, like French lancers and HRE gothic knights, they start out with advanced plate (conjured from the æther even when your settlement doesn't have so much as a leather tanner), and thus start out at top with no room for improvements.

The Italian factions -- Milan, Sicily, and Venice -- can actually use their Armour Factories. Broken Lances and Men-at-Arms -- mounted or dismounted -- can upgrade to advanced plate. However, considering that these are fairly mediocre units, an armour factory is still a dubious investment. As an Italian you might not want to recruit men-at-arms because you could recruit something more powerful, like mailed knights. Milan's Famiglia Ducale can upgrade to advanced plate. In addition, Milan, if going with the "all cities" option, might find dismounted broken lances their only real option for heavy infantry (no DFKs from cities, even as Milan), perhaps making armour factories more worthwhile.

The Moors are unique in that they have no use for any armourer better than the armourer. None of their units can be upgraded beyond the armourer level with heavy armourers, master armourers, or armour factories. (A lot of their units top out with a leather tanner.) They have some fairly armored units -- Christian Guard -- but they are stuck at their initial (fairly good) level of armor.

[Aeratus]

The highest level armorer has a higher chance to give your generals some ancillary items. So there is some use, but you're right that it's practically useless.

[Maklodes]

The highest level armorer has a higher chance to give your generals some ancillary items. So there is some use, but you're right that it's practically useless.

This is a little bit quirky. If your governor oversees the completion of any armourer better than a blacksmith, he has a 75% chance of getting fine armour. However, considering the cost, if you're trying to do that, it'd make more sense in financial terms to sell whatever you've got and then build a leather tanner, blacksmith, and armourer than to upgrade anything armourer or better. Although the leather tanner/blacksmtih/armourer route takes nine turns of building, while upgrading to a heavy armourer only takes five turns, and even upgrading to an armour factory only takes seven turns.

However, in terms of parking your governor in a city or castle with an armourer, rather than overseeing its completion, the standard armourer is the only thing that works in getting your general fine armor. A castle armourer has a 30% chance per turn of giving an immobile general fine armour, while a city armourer has a 15% chance per turn of giving an immobile general fine armour. Heavy armourers, master armourers, and armour factories actually don't have any chance of giving your general fine armour. Note that it's not just that they don't have any additional chance to give your general fine armour, on top of the chance from having an armourer: the chance you had when you had an armourer is lost. (This is weird, and I think it should probably be regarded as a bug, since it makes no sense, but that's how it works, as far as I can tell.)

UPDATE: The master smith ancillary is fairly similar, in that you can acquire one from overseeing the building an armourer or better (20% chance). However, the master smith has a (very low) chance of being acquired by sitting in a a settlement with an armourer or better (2% chance). Unlike the Fine Armour ancillary, you don't lose the ability to acquire one when you upgrade your armourer, but it doesn't improve either.

[PapaRosario]

I just wanted to add that your general parked in a settlement with an Armourer also needs 5 or more command in order to pick up the Fine Armour ancillary. And yes, that does seem like a bug that you only get it with an Armourer and not the higher level buildings. Very easy to fix though. Just change the "=" to ">=".

Edit: Or alternatively, you can make new triggers for the higher level smiths with a higher level chance and/or lower command requirement in order to get the armor.

Edit 2: Hmm this probably isn't the forum to be talking about modding. Ignore me.

[Maklodes]

I just wanted to add that your general parked in a settlement with an Armourer also needs 5 or more command in order to pick up the Fine Armour ancillary.

Thanks. I forgot to mention that bit.