In Warlords 4, whenever you move your units into a square already occupied by your other units, these automatically stack (of course provided there are at most 8 of them). In that game, I find it useful more often than not. I feel that most of the time you want to move your whole stacks and only sometimes you want to remove units from stacks. If my memory doesn't fail me, whenever I move a small stack into another small stack, I always do it to create a large stack.
This would also prevent misclicks that I, in my incautious campaigning in the fields, have made. For instance, I had a stack of my strongest hero and another unit, I clicked it and moved to conquer a neutral city only to realize I forgot the other unit behind and that my lonely hero got slain ingloriously. I simply forgot to add that unit to the hero, I thought they were stacked, but weren't... Imagine how mad I was at myself. Chances at winning that game are probably gone.