Niente means nothing! As such, I like to think of it as an effect, not a dynamic. It seems that a lot of people think players are computers and can really play at no volume.
What happens when a player sees a note with a swell that starts with niente? It is impossible for many instruments of the orchestra to start from absolutely nothing, so instead they try to come in as softly as possible. If you give a whole bunch of different instruments a niente start at the same time, they will all speak at different times. This is a particular effect and it may sound very cool, but it is not going to be the same effect as just giving them pp, and it will not sound anything like what your computer can do. Niente starts work best in strings. Some woodwinds can do it, like flutes and the clarinets in low and mid register (contrabass clarinet can do an amazing one). Trumpets and oboes, on the other hand, are not so good at it, and it is just impossible on the harp and piano. So if you want the whole band or large sections of it to start very soft but together, niente is not the way to mark it.
Just to make sure I was not making this up, I tried an experiment on a session recently. I gave two bassoons a niente start. It was a simple part, a G in octaves, top and bottom of the staff. There was a three-beat swell to p. Lets just say it was not pleasant and I will never try that again. On the next take, the players fixed it themselves, as I predicted they would, and just started pp anyway.
As I showed in this post, when you have strings start at pp, you get pretty close to a niente start by default. If you start brass at niente it will be a mushy start (but after a few takes they will all end up starting together anyway as they listen to each other). So that is when I use it. If I just want a really soft start with everyone together I use pp. Brass and woodwinds work differently than strings. As there are many players per note in the strings it does not matter if some people are later than others, and as previously mentioned, it can be a desired effect. Brass, however, are more often one to a note and so you want them to be together in most styles of music. I will use a brass niente in aleatoric or gestural sections when I don’t care if they start together, and I want them to be so tentative they can miss the start.
A redundant use of niente is at the end of a diminuendo. We often end a cue with a long held note that diminuendos. There is no need to put a dynamic at the end of this. Players will go down to nothing and often finish early, unless they watch the conductor for a cutoff. But dynamically it will be as close to niente as humanly possible, especially if it is just strings playing.
Here is why it is redundant to put niente at the end of a diminuendo. Watch a string section do a long diminuendo. The action is the exact opposite of how they start a long note. The bow ends up on a downstroke, so the last part of the note is played at the tip of the bow, where the pressure can be the lightest. They will not change direction near the end, they will slow down and then just run out of bow, all at slightly different times. This does get you a true niente effect, but marking it as such is redundant, because it is what happens by default!
I bet you $1,000 you will get the same result even if you leave off the niente.
One trick I often use to exaggerate this effect is to end with just the back desks. This gives it a cool spacial effect but also reduces the mass of players to give you a lighter sound at the the end. This also works in reverse, start with the back desks and add the rest over time.
I am not saying never use niente, just think about it first.