was/is the word, not essential
Don't get me wrong, I'm not saying that anyone shouldn't buy a whistle in a different key, but in just the last 10 years of sessions I can only recall once where a whistle player brought out a whistle in a different key. Everyone else just uses a D whistle (high or low). Granted, my sample size may not be huge, but I'm including sessions in Sacramento, Davis CA, San Diego, Seattle, St. Louis, KC, Baltimore, and Atlanta (that I can recall).
Now in our band we have a couple of players who break out different key whistle from time to time, but at session they just play a D whistle...or the bodhran.
It's just that in my 'circles' if you want to play along to the odd song now and again you'd better have whistles in other keys than D or C, namely A, Bb or F (low F is handy in those cases)