(About Escape from Velikiye Luki being scenario #6)
Not sure about this logic. Fighting Withdrawal is scenario 1, and it's far from a good learning scenario.
Of course the matter is highly situation dependent. Whenever you have lots of Guns, you have important units with little mobility, and will tend to use Prep Fire with them if you have any targets.
What I am sure of is that almost all new players I meet tend to do way too much Prep Firing than they should. Sometimes they'll use their entire OB firing in Prep Fire, often breaking some key units but then with no possibility of exploiting these breaks - the broken units will rout away, be replaced by others in the same or the next positions while the first get rallied.
When defending, it's often simple arithmetic that you'll be better off skulking and advancing back than Prep Firing - this will let you retain some measure of concealment, and Prep Firing will almost always be trading blows against much higher firepower.
So is it a universal truth that you should be moving and not Prep Fire? Sure no. But it's a very frequent one that the best path to victory will involve very little Prep Fire an much more movement.
Of course some part of the attacker's job is to maneuver so as to get into a position where he'll finally be able to Prep Fire a lot - surrounding that victory building, or breaking units that won't be able to rout away. But there's very few scenarios where this is likely to happen without a healthy dose of maneuvering in previous turns.