Sing is EXTREMELY situational, and cant really be thrown out in neutral game or for punishing, or anything really. Most of puffs combos are in the air, so you can't exactly combo into it. Considering it can be shielded, you cant apply shield pressure with it either. The only use I've seen for it is hopping up off of ledge, sing, and then regrabbing ledge while falling, which can work in some situation, like if a character is charging a slow move at ledge to edge guard you (Dedede's forward smash) but this is easily punished, so there aren't many uses for the move as it stands. Which, is sort of understandable, considering it gives you a free rest, but then again you can generally mash out of the sleep before the animation is over, so it's still not good. I've been able to catch some people with it when they aren't expecting, but that's not a good strategy to rely on.