I put no, but I don't mind it being done against me.
I try to make ONE balanced list and use it in every single game I play, for several reasons.
* I don't have much spare time and my girlfriend gets up me when I overspend on warhammer (especially if I cut into money meant for bills!) This way I plan my army carefully and I stick to it.
* I make my army tournament friendly. This means at least 40% troops, no more than 20% or so on any other category if possible, nor more than 20% on armoury, etc. I can then use it in any tournament against all comers and get decent points for army comp too.
* In non-tourney games who cares if you win or not?
* In tourney games it means I am using an army that I know inside out and back to front, instead of some cheesed up variation of my "normal" list that I have little experience with.
So for me this is clearly the most logical method. Not for everyone though, some people prefer to experiment constantly. I guess it's the difference between the sort of person who is on a quest to find the perfect weapon, and the person who decides instead to practice every day with the weapon that they have until they have mastered it (me).
I don't care if someone does it to me coz in a friendly game it doesn't matter if I lose, and in a tourney they CAN'T do it to me.
By the way, that first option on the poll, I don't think military comanders, modern or otherwise, have limitless availability in the sort of troops they have to fight a battle
It depends on which units are where and how fast they can get to the necessary theatre.
It's not like US officers in Iraq can say to the insurgents "Oh you've got some of those, well hang on, don't attack yet, I just need to order some of these up to counter them, 'kay?" So swapping out your army is actually very unrealistic if you look at it that way.