for example if you have a courser and 7 riding, you will almost insta kill with a heavy lance, but with a large warhorse, about half of all people. you can get less kills in the time you would with a courser, and the same counts for heavy armor.
I'd like to pull this quote just to emphasize my point for a second
ROHYPNOL, with one of the highest KDRs world-wide is a lancer who frequently uses heavy armor. He does well with light/medium armor, but he
really shines when he is in his high-cost gear.
Armor does have its drawbacks, but you really can't theorycraft your way into saying that armor is bad. If this were the case, you would would see the top 10 strewn with exclusively-low-armored melee and cav listed in the top 10 (or even top 100.) Heavy armor is clearly better than light/medium armor, even with its drawbacks.
Especially now, in its current patch, armor is even more effective at keeping you alive longer. I'm starting to gather that the new upkeep increase is partially done in response to the new damage calculations implemented.