The Dair, for me, can be use, in a specific situations, like :
- DDD is offstage and try to come back on the stage, do a Dair will often hit DDD cause he can't do a Bair or something, he is front of the stage, and even if he Bup, that will never hit Link. For me, it's more safe than on other character like Marth or MK.
- You are on Battlefield you try to kill the opponent, and force him to approach, you go on the plateform and throw some bombs, if you can read where he will go, you can try a Dair, because even if you miss the opponent, you got no lag on the Dair.
But for me, the Dair is generaly great offstage (on some character) and on the stages with good plateform, and he can kill really early on some character, just make sure that you will get no lag on the dair, otherwise, you will punish hard.
And for momentum, personally, i use only when i'm hit by an attack like Snake's Fsmash or his Utilt. The wrong idea would be of use for an attack like Fox's Fsmash. But for me, the Dair still a great momentum.
I know you know already these stuff, but i just said my opinion on the Dair.
And sorry if you don't understand what i say, i'm french (Belgium) and i never had a English lesson =/.