Exactly. Not having another uses of military MPs, unrealistic development increases from AI in combination with too high coring costs and the need to work around it by my tactic is bad design.
But you do have another use for military MPs. You said it yourself, you use them to pump manpower into your own territories... while simultaneously denying the AI the ability to do so, because you apparently lack the ability to mod coring costs down or something, since that seems to be the only complaint you have, is "Prague takes 900 points to core, this is unrealistic and bad".
Note: Abusing the pause mechanic to pump development into your own land, then turning it off for the AI, is an exploit and shouldn't be taken seriously as a reason for "bad design". If anything, it's just you cheating. Might as well use the console.
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