Let me do a wild comparison of militia-knight to some other knight, lets say terminal-silver, because its easy.
If they have some other Kinght, lets consider +2$. They play it, so they have 2 cards an no Action remaining, but have +2$. That means in direct comparisson they have 2$ + 2 cards (selected out of 4) to buy something, and you have 4 Cards (selected out of 4). Which is better? Well, I would say most times the former is the better.
For buying a $5, $2 + 2/4 cards is great, you have a good shot at getting Silver+Copper here. 4/4 cards is much worse, especially since you've been investing your $5s on Knights.
For buying Gold they are about even, $2 + 2 Cards is kind of hard to get the first or even second gold, but 4/4 is really hard, although with its high variance you will eventually do it if the Knight trashing holds economies down for a while.
For buying Provinces, $2 + 2 Cards is probably better, as you "only" need to get two of those Golds, while 4/4 you need two Golds + a Silver or 2 Coppers. Also, by the time you are greening, discard down to 3 becomes not so problematic.
So, all in all, I don't see that militia-knight is as dominant as the ratings might suggest. Is there something completely off in this analysis?