Well, I guess it's a matter of playing style, but it isn't hard to win battles, or even to pretty much avoid them. If you do occasionally lose, it's seldom catastrophic. Even when it results in the AI capturing a province, they always immediately march off and let you retake it without any hindrance.
What you can't avoid is taking fortresses, and mostly being able to take them fast is invaluable - quick wars avoid war exhuastion, especially through not getting caught in a long series of BB wars without being able to reset the war exhaustion to zero. So you can much more easily sustain an empire of all the religions, if you've got good seige ability.
So when I get a new leader, the only thing I really take any notice of is the seige value. It's worth having a war just to make sure you get some use out of a siege-strong leader, I wouldn't do that just for one with other good stats.
I can't remember any with seige 3 (not that I'm denying they are there). Who are you thinking of?