I've only used b-throw if my back is right against the ledge, other than that it doesn't have much use.
D-smash is an okay move, if anything it just adds to what your opponent has to take into consideration, which makes his job of defending harder. I find it especially useful against MK (or light characters in general that have multi-jumps, or have great recovery), even more so if the first time I use it is for killing purposes. It usually won't ever be stale, it has great vertical killing chances, and landing a tipper is arguably easier.
F-smash is better overall, but if your opponent is looking for the F-smash (not jumping as much), then it increases the chances of D-smash landing, as opposed to being dodged/punished.
As with Marth's move-set, everything works well in tandem, and forms a very complete system. Leaving out one part of the system only decreases its efficiency.