I think just being at war is enough to justify the mutual struggle bonus, because there is always the chance that an SOD will arrive at your doorstep. Most of the time doing nothing doesn't cost you, but last night an AI had just taken another AI's cap. I figured they were fully engaged and agreed to join when asked, only to be surprised a couple of turns later when a stack of 50 units arrived at my doorstep. I found it silly that they were still losing cities but decided the stack would be better used attacking me. Just silly.