Originally Posted by GrimzagGorwazza
Even assuming that whicheve shot of this selection is able to auto kill whatever it hits, you're still only averaging a single casualty here. Unless the charger has set it so that his commander is in the front or his front model will be the only one who makes it into charge range it's still not going to deter your average space marine assault squad, ork mob or csm unit, heck the sarge has as much chance of cooking himself with the plasma as he does of hitting the enemy...it's just not enough damage to be considered game changing unless you can spam the numbers you need.
Overwatch is important for two reasons: doing pre-assault damage to an enemy squad to weaken it for combat and doing pre-assault damage to an enemy squad to deny a charge.
With that many high power shots, you'll average about a kill every overwatch and the value of doing better than average increases much more rapidly than the consequences of doing worse than average.
With random charge distances, charges can be divided into two groups. Important charges, where the assaulting player has moved to close range to mitigate the affects of chance. And Hail Mary charges where the chances of making it aren't good, but the benefits of trying make it an option.
In the case of the first, it is more important to do damage than to deny a charge, but against termies it can deny a charge even at close distances.
In the case of the second, you're likely taking a free shooting round at the enemy unit and if they're lucky with the dice your overwatch might be the difference between base contact and one inch outside of it.
I'm not saying overwatch will ever be gamechanging. All I'm saying is that if you pack enough firepower density into a squad it can be successful in enhancing a unit's combat effectiveness and even preventing the unit from being destroyed in combat.