[Baalz]

Without delving into the whole 'MA Ulm is worthless' discussion (that I strongly disagree with) let me address the original question about black plate. Just like almost everything in this game when it's usefulness is situational, but I find it quite useful for the simple fact that on a per gold basis the black plate infantry is some of the best meat shields available in a lot of circumstances. You throw a legions of steel on them and they do ignore all but very heavy attacks, and there is very little that can take them down without several hits apiece. Just try and face them with spears or light lances. This is a 10 gold unit.

Any time that you are gold constrained (a state I find myself in a good portion of any game) it's black plate all the way. Ulmish infantry is not there to deal damage, forget about the pikes and weapon choices as none of that is going to be terribly effective against anything but indies anyway. Ulmish infantry has one role - stand there and get hit over and over again, and it's a job they do well and a job the black plate boys do even better.

If you take production-3 and place your castles intelligently you'll have 3-400+ resources and with no real expensive stuff you should be able to crank 'em out steadily. Just realize that they are there to get hit, not to kill your opponents and bring something else to deal damage. Even as research progresses black plate infantry remains cost effective for this roll for quite a while because of their cost. Sure, laying down some thunderstrikes is gonna explode that black plate infantry but I'd much rather have that falling on my 10 gold infantry than 70 gold Vans. The thunderstrikes will end soon enough and at that cost I'll still have infantry left.

What do you do while the bad guys are flailing away ineffectively at your plate? Heck, Ulmish smiths are custom made to be effective combat mages. Real cheap, so you can have a bunch of them and not cry too much if some fall. Summon earth power so they've got reinvig. Destruction + blade wind = uuuuugly. Magma eruption (black plate guys are tough enough to even take a bit of friendly fire without dying). Earth meld those pesky water blessed troops. The Ulmish knights also can pack quite a wallop, just script them to hold and attack so they ride in on the flank after everybody is tied up on the infantry.

Heck, even in the late game the black plate infantry can be fairly effective, 30-40 of the flail guys backed by 2 smiths who cast respectively army of lead/gold + weapons of sharpness along with whatever other buffs you like is gonna be a *damn* effective squad for the cost - you can field dozens of these all day long without slowing whatever your main strategy is (bonus your boosted smiths can also cast petrify once they're done buffing the troops - fear no SCs teleporting in on you).

As far as the forgeable black plate it has one very important feature (same theme as the black plate infantry) - it's dirt cheap. Pesky opponent earthquaking your communions? Put black plate on all your mages and their survivability goes waaay up.