Stacy laid it out well.
Salt heats by conduction, which is the most effective method to heat steels for heat treating.
The big project I've been working on lately, the Joe Paranee X2 Fighting Bowies, make for a great example of why salt can be so nice to have. The blades are right around 3/8" thick at the ricasso, yet taper drastically to a nearly needle point, and they're nearly 2" wide at the widest point.
If you heat that type of blade geometry in a forge, you'll damn near burn the point up before the super thick ricasso even has any heat in it.
In theory, if you soak it in a kiln that is set to not overshoot your temperature, the ricasso will eventually come up to the same temp as the tip without overheating. But I can tell you from first hand experience, that in application that rarely works so well. That is with what most knife makers have available to them. My Paragon Kiln is a KM24, I think it cost about $1400-1500. It holds temp pretty well, but it can't hold a candle to the rapid heat transfer and precise temperature control of my salt bath.
Now if you look at the vacuum furnaces that a place like Peter's has, then you have a comparable "kiln" but it will cost as much as your house.
And speaking of them, Peter's actually uses molten SAND baths for heat treating a lot of simple carbon steels.
I view salt bath safety a lot like any tool. You have to respect it, and understand what it is capable of... then you weigh the risk to benefit ratio and decide for yourself if it's something that's worth having in your shop. If it's something you don't understand and are afraid of, then you should definitely NOT have whatever tool it is.
