1) The dair is still affected by momentum. If you get sent flying upwards and use dair, you won't fly down as fast.
2) If you get sent flying up fast enough with a low hitstun attack, you can use dair to stay in place in midair, or even fly upwards while using the dair.
3) You can use this to save yourself from upward kill attacks. As long as the hitstun of the attack you're being hit with is low enough, you can use your dair to lose upward momentum, and save yourself.
4) You should never attempt to spike with the dair, it's more like a recovery move for upward kill attacks.
5) Even if you do manage to spike with the dair, it's only a meteor and can be meteor canceled. You will always die first if you manage the spike.
6) Spike with B-down, or footstool with B-down. It's much more effective. B-down footstools gives you extremely good aerial mobility and distance compared to that of a normal footstool.