Because math makes me hard!
The Binomial distribution is indeed the right distribution for this sort of thing as it handles the probability of k successes in a series of n yes/no experiements (in our case, did the wych die yes/no).
The probability of k dead wyches from n boltershots fired is given by,
where p is the chance of a wych dying, i.e (1/6)*(2/3) 6's to hit, 3's to wound for overwatch when the wyches have no cover or FNP.
So here are some plots for the chance of wyches dying from 10 bolters overwatching (so 20 shots)!
The x-axis is number of wyches killed from overwatch and the y-axis is the probability of that many wyches dying. The calculations were for 20 shots (i.e 20 dead wyches), but I only plotted up to 7 because as you can see the probabilities are pretty much 0 after 7 (although not exactly 0, there is a very, very, VERY small chance of 20 shots killing 20 wyches). The different colours are for different situations as follows;
red - normal overwatch, p = (1/6)*(2/3)
blue - overwatch when the wyches have FNP, p = (1/6)*(2/3)*(2/3)
yellow - overwatch when the wyches have 4+ coversave, p = (1/6)*(2/3)*(1/2)
green - overwatch when the wyches have 4+ coversave AND FNP, p = (1/6)*(2/3)*(1/2)*(2/3)
As I'm sure anyone can see the chance of you not loosing any wyches to overwatch is pretty small, <50% even at the best of times. I really think overwatch hurt wyches really bad. For a discrete distribution like this you can for super(-duper-ultra)-simplicity sake (sorry if there are any mathematician here) you can view the highest probability and expected value to coincide (so 2 for red, 1 for blue etc.).
EDIT: I forgot my point; If you have a small squad (5) wyches charging 10 bolters the chance of loosing the ENTIRE SQUAD is higher than not loosing any...
Comparing this to what happens when marines charge into those 10 bolters,
you can easily see there is quite a big discrepancy there, you MIGHT loose 1, but most of the time you wont... I personally think overwatch hurts DE more than any other army as we have relativily few (as compared to say ork boyz or gants) fairly squishy (compared to say assault marines) dudes trying to charge stuff.
This is what entertains me on a slow friday evening!Tl;dr OVERWATCH HURTS IF YOU'RE WEARING SKINTIGHT SPANDEX