It depends on the position of the marsh, your city, your army, and the enemy's army. If the marsh leaves room for your army next to the city while forcing the enemy to either walk on it or make some awkward manoeuvers, then it's great. If it's behind your city, it makes no difference, or may even render your reinforcements slower.
In the 1066 AD scenario, between the two northenmost cities of England, there are two rivers dividing 6+ hexes of marshes, surrounded by some mountains and hills. That patch of land was a nightmare to cross, since the defender had a couple of crossbowmen and a catapult waiting for me on the other side...