I feel like that for the good of mankind, it would be a heck a lot easier and safer to have the missles waiting in parking orbit over launching said weapon straight to it on the notice of an impactor. Besides, it would take atleast four years to get one of those missions ready, where as having them in standby would allow for a quicker, more on the notice launch. And if you have a malfunction with the deflector on the way, you can still launch another one to defend from the asteroid. It's like a person fending off an attacker and coming with just a muzzle loader. If he misses or the bullet is stopped, he has to reload his weapon, whereas if he brought, say, a revolver, he would be able to fire another few shots just in case. Be prepared.