Thanks for the link, Sullla.
OK, my example was bad. I'm used to SMACX combat where healing in the field takes way longer (and you can't completely heal until late in the game). By the way, if you retreat completely then some of my catapults might stay alive, and I would have made some more, so we're back to zero with me being ready now.
Of course I've invested less in the military since I didn't start producing those supplementary units before I saw you show up.
Whipping will have to be carefully applied of course, but as a defender I don't need as many units as you to stop you. This becomes even more true with forts.
And in Civ4 there's no cost for retooling production to something else.
Anyway, in the later game, you're not going to keep more than say 6 units per stack, unless there's no space to move, if you know that there's artillery defending (and the more advanced the game becomes, the worse it is since artillery evolves and does even more collateral damage to more units).
Also, the surround and destroy mod emphasizes smaller stacks.