I thought this idea sounded pointless at first but then just last night I was playing the demo and this happened to me. As Portugal, I was running the game speed fast and Spain called me to arms. I was unable to pause in time to respond and it broke my alliance.
Back in EUIII this simply wasn't an issue because the game wasn't fast enough to cause the problem.
EUIII was a bit slower, but it still happened to me. I would just be doing something completely different and miss the CtA at the last second. It sucks. I think that making it an option in SP is perfectly viable and should be allowed.
I don't understand some of the people that play MP on PC games. It seems that when they have a lack of features in MP because a game has been developed in a more complete way for SP they cry and whine. But if any one wants something for SP that might not be ideal for MP games they freak out.