Depends on how many you have. There are armies that routinely have their infantry carry one siege engine per unit. For younger characters it doesn't have too much effect, unless you're traveling through mountains or on very bad roads. But when you get the unexpected chance to attack a fortified but lightly defended townsland, it is very nice to have 5 or 6 siege engines your enemy didn't expect you to have.
You are right but it can make a difference. it's not so uncommon in a prolonged war to routinely cross destroyed regions with painfully long travel times. Sometimes even a couple of siege engines per unit can make a difference in the turn you can reach a given region.
That's not so relevant anyway. I anyway completely agree with the beginning of your post and the importance of siege engines in the combat game.
