Kinda depends on who you're using. As a Dictator main, the only time I can ever effectively use FADC is to cancel scissor kick pressure into tick throw or just to do something a little different to keep my opponent to getting too comfortable with what I'm doing.
Something like: j.mk -> cr.lp, standing lp, cr.mk xx lk scissor kick xx FADC the first hit -> dash in for the throw.
Or j.mk -> cr.lp, standing lp, cr.mk xx lk scissor kick xx FA cancel the first hit -> hold to charge the FA to level 2 and let it go -> dash in -> if it hits, do some crappy Bison bread and butter into FK scissor kick, otherwise throw or cr.lk and continue scissor kick pressure.
For a number of characters, there isn't a whole hell of a lot else you can do with it.
For the shoto squad, I'd say it's pretty damn important as it's the only way of comboing into an Ultra off the ground outside of the corner. With Ryu, in big punishment situations, try working into your game something simple like cr.mk xx fp shoryuken xx FADC -> Ultra.
