Undiminished dair actually has a surprising amount of killing power, and it's always helpful to gimp with it if they're recovering below you. As mentioned in a previous post, jumping and dairing is a pretty good idea to get into as well as it helps your spacing and can even bait attacks out of your enemy.
Nair's only purpose is a surprise kill move due to its power yet its very short range(considering MK). You could FF-nair(though situational, I'd bet), but you want to connect with the first hitframe of the attack otherwise the knockback will be teh sux.
Though I feel like I'm being redundant and just repeating everything everyone just said.