I mind reading in a gundog book many years ago, his theory was that silent whistles are not ideal esp in novice hands as they might not be blowing the correct signals , or even the whistle might not be working, u'd never even know.
Have to admit i've never used 1 but do see odd folk with them and bizzarely often have both silent and normal whistles on there lanyard, thy do dabl in FT spaniels so possibly trained to both and u can whistle a tune on the silent and no one rally knows ur dog is disobeying u.
I'd agree with others makes no odds which pitch really and just buy a few and leave them a few likely places.
I have some hanging in kennel, back door with jackets, pick up dash and the emergency 1 in my game bag.
I mind picking up a duck flight after work 1 night so was rushing to get there, took the guns up in a different motor so forgot to take whistle, was a long night.
In the past i had some random wrong pitch whistles i must of bought by mistake and found if training a dog 1-1 it still responded fine to wrong pitch whistle, could well be a different story on a busy shoot day thou