Basically we're counting on the idea that we can out-stubborn any lowlander attempts to use the area south of us. We're betting that the lowlanders won't have the appetite to dish and take the level of punishment we'll inflict to keep them out of our borderlands. It doesn't improve the strategic situation, but it keeps it from deteriorating, which is better than nothing.
But Azel, we kinda did the whole assemble a warhost to take land permanently thing, and we got tricked by Brushcrest and nearly were swindled of our lives in a futile struggle against Makar. The system change forced us to spend a few turns building up our food production. I guess you're right that we could do more than just raid the herdsmen, we could follow them back to their homes and inflict a more decisive blow there. Oh, I see what you're getting at!