How are they not side quests if they're optional and many never see them? I liked that little addition, don't get me wrong but I wanted to see the army we supposedly got. Where are they in Haven? Where are they in the Arbor Wilds? There are like 12 random allies trickled along a forest path, that doesn't give you the impression of an army at all. Even the preceding cutscene showed only small groups or handfuls of scouts. The entire setting and plot of the game doesn't change based on Sampson/Calpernia, what are you talking about? Do you mean the plot of the one side quest? This was not enough to satisfy me.
They're a divergent main plot. That's the thing. A side quest is a quest that is independent of the main plot, and thus is played "on the side".
Samson and Calpurnia are direct results of an earlier choice in the main plot. As a result, your main quest proceeds differently with a different enemy force attacking Haven and a different enemy force assaulting the Arbor Wilds, both main quests. There is a side quest for each of them to learn more about them, but that's different.
It's an entirely unique setting at that point. You can't dismiss that.
It's understandable to feel that the side you picked is "useless" after the Breach besides some points in the story (isn't that the case with all DAO choices anyway? In DAI you at least can interact with them throughout the game and use them in the war table as a commanding officer).
The thing is you end up getting attacked by the side you DIDN'T assist and they become the main antagonist's force. So the consequence of the choice is there.
Also I don't understand the battlefield part. Arbor Wilds played out just like the Battle for Denerim, except you were racing Corypheus to a common goal rather than attempting to kill him and your combatants differed depending on what choices you made previously. Making it a battle like Ostagar would make little sense given that it wasn't an outright melee.