In a life threatening situation against probably a bigger, heavier attacker you need to be able to deliver full power atemi to vital points, if you are using atemi in your aikido to defend yourself. Even on soft vital points, your probability of success increases with how hard you can strike those targets, all other things being equal. To deliver the full power strikes requires training at full power strikes. To do the training with a heavy bag (without gloves) or a makiwarra requires some degree of hand conditioning.
I've seen posters disagreeing with many of Daniel's points.
I'd just like to butt in and completely agree with what's in this quoted paragraph.
Personally I don't use a makiwara or (intentionally) rough contact surfaces, that's my choice. Skin-toughening sometimes occurrs as a sorry side-effect for me, not an aim.
I also don't use spear-hands or unshod toe-kicks.
Apart from that : condition away! Have at it! If you don't possess a seriously tested knock-out-force strike you're missing a BIG piece of what Ueshiba had in his arsenal.