I reckon knights are good as they are. Ignoring the fact you can bypass the pop rate with money for promotions, knights are roughly equivalent to special A/B in terms of power. That they have the same pop rate makes sense to me.
Shadow, think of it this way. They take a while now, but get a better city and before long you can have a decent force of knights. They're not a unit you send out in the hundreds, but an elite force to form stacks for striking or guarding key units.
With the dawning of each new day, my evil machinations inch me closer to world domination. And also breakfast.