"Very well, turn your back on us if you must"

Pretty much what John said. If I'm going for a DV the I'll pledge every city state I meet. Starting at 25 influence is nice. After I've made friend/ally with then, then I drop the pledge sine it does me no more good, and even a minor diplo hit can cause problems when added to things like when you're ally with CSs that someone else want's ally with - You built wonders we want - Borders close - Covet Lands. If you get 2 of those, or worse yet, 3 then it starts to add up.

Besides, I actually think that aggressive Civs intentionally go intimidate CSs that they know you're protecting to try and get you to declare the war so that they don't take such a large warmonger penalty. Or diplomatic (Yes Alex, I'm looking at you) go intimidate CSs to try and get you to lose the 20 influence. Maybe they aren't that smart, but I like to think they are because it makes me feel better about all of their other failings.