While it's worth considering, the idea of "don't make warfare too easy by preserving all of your people" strikes me as placing an abstraction over people's lives.
Except the 'abstractions' are the lives of the people massacred in the name of your conquest machine to establish [insert crusade here] in order to further your ability to conquer and crusade. The only people's lives being saved are your own- everyone else is dying as much or more.
Unless we deny that the lives of other people have value- which, to be fair, you have pursued on more than one occasional over the years in your various supports of mass oppression and genocide in favor of your favored causes- then there is absolutely a moral case to be made against war, even if the only people who die aren't your own. Especially if it makes you more likely to be willing to engage in them. Practices that encourage these 'cheap' wars, which are anything but cheap to the people being massacred and re-raised by your undead legions, are directly encouraging a greater loss of life by both frequency and potential scale of conflict.
It just needs to be leashed with the proper philosophy, which is, I assume, why Nevarra hasn't already conquered everything in sight with armies of the dead.
Or, alternatively, necromancers got crushed and everyone else instituted a taboo on what was considered acceptable warfare.
Which happens quite often, and is why we don't go around slinging WMDs against each other in the name of saving our own soldiers lives. The nasty thing about 'advantages' isn't that the first one wins everything, but that the promise of easy and clean victory tends to disappear when others copy your strategy. A necromatic army is only an exclusive advantage until everyone else starts raising their own- at which point, everyone's advantage is everyone's increased suffering.
A war of the resurrected armies isn't a bloodless affair- it's a mob brawl with a constant need for bodies to supplement, and then become, your necromatic hoarde. No 'proper philosophy' is going to restrain everyone in a time of desperate war- they'll look for whatever bodies they can, wherever they can. Your own armies. Your enemy's armies. Your enemies people. Even your own. Criminals and deathrow inmates at first, of course. But then, going by what you've posted in the past, dissidents and ideological enemies of your favored classes will soon follow, followed by who knows what else.
Normalizing necro-wars would be a great thing for Thedas, so long as you think that making genocide an actual logical strategic advantage is a good thing. Is that village beyond the veil strategically insignificant, or your next much-needed battalion of skeleton archers?