The thing is by doing knuckle push-ups your wrists will only become conditioned to the one position, by doing forearm curls and reverse forearm curls your wrists are being strengthened as they go through a full range a motion, so if your wrists do give as you hit the bag (which happens to nearly everyone at some point no matter how long they've been training) you may have the strength in your wrists to not bend to a point of severe damage after you've punched.
Since I started wrist training I have noticed quite a difference in wrist strength in regards to punching, though in regards to punching I do a fair bit of reverse forearm curls because they work the forearm extensors that are being worked against when the wrists are bent on contact during punching.
Forearm training is a big thing to me, I just like it and the feeling of power that comes with it.
Plus I personally find you can work the forearms alot more than other muscle groups, perhaps because evolution somehow knew we were going to use them quit often so gave them massive endurance in comparrison to other body parts.
So whenever I get the urge to train and all other body parts are fatigued I do some forearm work...my 'other' hobby besides MA