[Baalz]

Yes, just as your enemies know what your weaknesses are, so do you and you should plan around them. With magical support on one or both sides it's a rare fight that lasts more than three or four rounds of the infantry slugging it out - you rarely need to worry at all about your infantry passing out from fatigue. What you need to worry about is them collapsing under the initial charge, being decimated by artillery/archery before they engage, and being flanked. If you can buy the time for your mages to go through all 5 spells you've scripted, and maybe lob a couple more on their own you've already blown 80% of your load - generally the battle's decided at that point.

As to the lighting/poison/whatever argument - I don't buy it. Your argument is that there are some situations which black plate is no better than chainmail. No arguments, my retort is that they cost the same gold which is usually the limmiting factor on how many you can get. At the end of the day I'd rather have 40 black plate infantry than 40 chainmail ones because there are lots of situations where the black plate is better.