Is black plate worth it?

[Sombre]

Well obviously black plate is better vs archers. But is it that much better than chain + shield that you want to use an extra 10 resources on it? Hell no.

And in my experience resources are a serious limiting factor in the early game. Gold is also a limit, but when you're trying to grab indies and possibly muscle another nation, high resource cost is a serious factor. If you're taking prod 3 then that's even more that resource costs are limiting you, actually taking away design points. After the early game, these troops basically become useless anyway, because battlemagic renders them largely obsolete. Sure you can use them as a walking shield, but there are better units for that purpose, including (imo) the chainmail guys, who you can replace quicker.