Not only that, but if you think of it in real life terms, the generals would have sent out scouts to check out the battlefield ahead of time, in order to send in the most appropriate troops.
I don't think too many generals would go willingly into battle blind to the terrain.
I could be wrong though.
It is a rare case indeed that a place of battle was pre-arranged and participants had the luxury of picking and choosing what to take along for a specified situation or agreed upon limit.
I can understand Dave's dilemma. One just has to rationalize it away as 'now I'm stuck with this situation.'
When I am faced with such, I just figure that the enemy is hoping I won't attack but I must, because in the 'meta event' my opponent is going to get stronger and I weaker, thus I must force the issue. There are a multitude of historical events that often had similar precursors.
It is analogous to the Meta Event that leads up to a particular battle/conflict that sets the tone and temperament for the major 'actors' / players. What leads them to make the decisions they do.
IMO, it is more 'realistic' to have your Army roster already built and then to use on on whatever battlefield you find your enemy. But this should allow some sort of 'Meta Game' system for 'motivations' and allowances to decline a battle and maneuver.
I have sketched out a campaign system concept where the over riding factor is a set number of points for the whole, but you must parcel out sub forces to achieve the goals.