I was never sure how much this actually reduces the fines, but does really good at washing them to the bottom of the coal pile. The fireman still has to deal with them, in one big batch, and throw them somewhere, usually in the firebox. I always experienced that it would gradually decrease the coal space by allowing the fines to accumulate at the bottom.
Back in the day, we would do this now and then on the D&S in Durango. At that time, it made a big mess and tended to loose some good coal to the ground. Again, it seemed to concentrate all the fines to the bottom of the coal space rather than removing them and decreasing coal capacity..
I was always the fireman that kept the coal wet with the squirt hose so as to keep the dust to a minimum. The squirt hose was also good for eroding the fines in a coal space that 1/2 to 1/4 full and allowing the good coal to fall forward. Any wet/muddy fines were thrown out the gangway so as to increase coal space for the next load.
That was my feeling during my time, but the current C&TS folks may have a more positive take on all this.
MDR