[HoneyBadger]

It's possible that it would work as you say, and not make turns noticeably longer (although turns do take quite a bit of time for the computer to process, and I've been using reasonably fast computers in the past), but I still don't quite see the point of it fluctuating.

Even if it were based on Dominion levels, you're still talking about military assignment (midieval though it may be). I'd personally get frustrated pretty fast by a PD that was noticeably weaker than I expected, after investing quite a bit of gold in them. Battle itself is chaotic enough-why have stationing of guards be yet another variable factor?

As far as the "viability of extremely high PD" argument, it really doesn't matter, because it's possible to have extremely high PD in the game, already. The difference between 125 and 150 is pretty minimal (25 more infantry by late game? Compared to what you could have done with the gold you spent on them?), unless a lot of the other suggestions I've made were also added.