I find myself autocalcing sieges more and more lately. Not in order to reduce losses (i actually find the way it applies losses equally among your troops to be fairly annoying) but because i find them to be a bit of a chore.

Camera control is really tricky around the walls, it's really hard to get units deployed where you want them inside the city, and, at the end of the day, its just not difficult enough (archers on the walls should be cutting you to pieces but theyre not - as discussed in that recent thread).

Also they tend to take ages - after the initial breakthrough there's often that long dragged out bit where the two bunched up armies slog it out along one road. In theory i should send units around the back to flank them but it takes so long to do that i can never be bothered.