Harmony - Troop Symmetry

For example:
It's not always beneficial to attack immediately. Sometimes, it's more advantageous to focus on recruiting troops: gather your forces, defeat a neutral unit, and you're in a better position. And sometimes: invest money in development, and you'll be in a better position than if you just got the reward for killing a neutral unit.

The same applies to a strategy involving resurrection. It doesn't eliminate losses entirely; it only reduces them by half. So, significant losses still remain significant.
Essentially, resurrection provides the opportunity to defeat stronger neutral units earlier with the same level of losses. For example, defeating 100 hypothetical Hydras without resurrection might result in the same losses as defeating 120 hypothetical Hydras with resurrection.

Moreover, even if we accept this point of view, developing in the single-player campaign is already a deliberately chosen strategy. Why should it be applicable to all decisions?

Added 4 minutes later

Building up a town is something you do in the first month, not in the mid-game.

In the first month, the mid-game, or even the end-game often begins. The clear end-game is when everything is built, and there's nowhere else to expand, and you just accumulate troops (the most tedious part).