Here's one other consideration (I think) when discussing a standing army vs. the PD. You don't pay upkeep on PD, and when PD wins, all losses are regenerated. Actually I'm not 100% sure on either of those, but I'm like 99% sure
Depending on the length of the game and the number of PDs and size of standing armies the initial PD cost vs. the upkeep will have different break points for where its effective and where its not. Thats still an oversimplified view though, as often depending on terrain, your reaction force will still be slow to get anywhere, and sometimes even supplies will be an issue for them. Then again, the reacaion force will probably be stronger than the equivalent amount of PD...
Its a question that isn't quite as simple as some make it out to be. There are circumstances where its simple, but likely more where it isn't so cut and dried.
My 2c anyway
