Man, I really need to get on that Skype Chat, perhaps once school ends haha. Anyway, from what I can tell, slow Dair is an artifact left over from Brawl when attacks used to be able cancel hitstun really quickly. In that game's case, using an attack would also greatly reduce one's momentum very similarly to how Smash 4 has that big deceleration happen about a second after you get hit, although more pronounced. However, characters with stall and falls like TL's and ZSS's would of course be really difficult to kill if they were simply able to not only cancel momentum with those attacks but completely shift it almost immediately. Thus, those moves were programmed in such a way that in the case of the character taking knockback, stall-and-falls would simply keep whatever current momentum that character had when the attack was initiated. However, it wasn't programmed perfectly, and this has resulted in characters being able abuse the momentum you have right as the knockback ends (specifically the part where you are hanging in the air for a split second). At least, that's what I think is the case (I looked it up and couldn't find any specifics on it). I could be wrong; Lavani would probably be a better person to explain this than I.
This was carried over into Smash 4, and this combined with Greninja's excellent airspeed makes Dair stalling a legitimate tactic for stifling momentum and helping get away from an opponent's pursuit. Basically, if you take high enough knockback, just double jump(fastest way to escape hitstun) and immediately Dair. It's not something you can activate freely, but it has its uses.
