I use it too, and I have noticed that it can be very effective for edge guarding. Simply knock your opponent off of the stage and then position yourself so that you are near the edge, breathing fire towards your opponent. If they hit your fire breath, they don't recover their used-up jumps, so they will plumet to their doom in a few moments. Obviously, this won't work well if your opponent can go over you, but it's something useful to keep in mind that may set up a hit with one of Yoshi's fire balls.
I wish that you could tell Yoshi to breathe his fire, though, as that would make it a lot more useful.
I also agree with Eddosan that running into people with FS Yoshi can be quite useful.