[Mightypeon]

I heavily disagree that map move does not influence battle performance.
Map move defines how many troops will be present (some special situations in which you have only 1 fort and it will be besieged soon excluded), and will often be more important for "massing" purposes than difference in gold/resource cost. As Tamerlane said, "it is better to be on hand with 10 man than absent with 10 thousand".
If you are reinforcing your advancing army from 2 castles, reinforcements of Sapper will eventually catch up with the army, reinforcements from Arbalests much less so. For 2 Ulmish castles, this can easily be 30 Sappers more in the important battle.
In addition, the speed with which you can storm castles influences the size of the relief force, and the size of the garrisson if the enemy has summoning abilities.
If you are sieging someone, with each turn, the size of the possible relief force (or rather, the area from which a relief force can be recruited by the enemy and arrive in time) increases.
However, if you have enough Sappers to "one shot" the castle (and that number is not very high), the enemy has to stop you with what he has at hand right in that area, and any blocking summons he may conjure in haste will not be organized and thus quite useless in the castle battle. This reduces castle taking casulties more than people commonly imagine.
Also, if you are attacking someone, chances are that there will be viable and important targets for Sappers on the Battlefield.

Dominions 3 has several resources, and a successfull strategy is commonly aimed at exploiting the limiting resource of the enemy.
In Dominions, those resources are gold, gems, resources and time. Sappers are highly efficient in "time" warfare, as Map Move 2 gives you a greater freedom/ greater recruiting area to react to a sudden threat, and secondly, they directly reduce the time the enemy has in responding to threats of your own.
There are situations in which time is not that much of an issue, such as getting rushed with only a single castle, but time, or rather, a turn advantadge will never go "out of fashion" as ressources and to a lesser extent Gold do.