In the game, it was laid out at deployment, so indeed it was there from the beginning. The BGs were indeed also in Difficult going, and were moving their full 4 inches per move, so 8 inches per turn with a march using only 1 general. This is really a bit more effective if the army using this tactic does not have the PBI, and so can move and march before the other army can react, or throw their own skirmishers forward.
The main factor I think, is that the MF were able to clear the terrain a full turn before they would have had they been only moving 3 MUs per movement. This may not seem like a lot, but there are times that one turn is crucial. Having played a good bit and done so well as you have, I think we can again agree on this point.
Again, I don't see this issue affecting every game, but it is in there, and should be addressed. Right now the rule goes completely against the author's intent according to RBS and few others. If that's truly the case then I'm glad it's going to be fixed.
So the only time I saw it done, it was from the start of a game so there was plenty of time to deal with it - especially given they were using it to get through a marsh. Some decent LF bowmen could cause them considerable chaos.
To this point, certainly if you know in advance all the parameters, then surely you would bring the proper army to deny the use of it. However, life's not always so perfect.