I might agree with that if human civilization as a whole was much more rational than it is now.,(especially the institutions that deal with political and military power - this includes organized religion to a certain degree in most places).

If I believed that warfare would only be used to attain noble goals that nothing else can reach (a "cause worth killing for", as you say), then yeah, if it's worth killing soldiers, it might be worth killing civilians too.

But right now, it seems that war is mostly about small politics, personal status (both for dictators and democratically elected leaders), xenophobia, and money.

I feel that if civilians had been a legitimate target in most recent wars, the outcomes would only have been worse, not better, and so I can't support it.