Sing is completely useless except for the lulz and on stages in which sing canceling is easier than on FD. I know that Japes platforms, as well as assorted parts of other stages (RC, Orpheon, Castle Siege, etc) allow for this as well. It should be noting that sing-canceling is just a form of lightstepping, which can be done with any move really.
Sing's only even slightly practical use is if ledgecanceled, and even that is situational as ****. Don't use sing in a tournament. Really don't use Jigglypuff in a tournament either unless you're up against Diddy. MK and Kirby are superior to Jiggs in every (high-level competitive) matchup EVEN IF THE PERSON IS EXPERIENCED WITH THEM... except for Diddy, who I genuinely believe Jiggs has the advantage against. You can also grab-release Ness and Lucas effectively and plank like. Well, Jiggs is an amazing planking character. The very few occasions I've played normal brawl Jiggs, I've won by timing out matches.