You need 3+ units to take advantage of it.
Say you have 3 units of firewarriors with a shas'ui and a markerlight.
First unit fires at normal BS.
Second unit has a 50% chance of firing at a higher BS.
Third unit has a 33% chance of firing at BS+2. (thats better than 25% with no cascading)
I think the best way to make cascading work is by using 3+ units with 2-4 markerlights + other weapons in them shooting at the same target.
Example; (Assume all squads sitting around 20-24" away from target)
12 man fire warrior squad, shas'ui, markerlight + target lock, 2 marker drones, with an attached fireblade
12 man fire warrior squad, shas'ui, markerlight + target lock, 2 marker drones
8 man Pathfinder squad, 3 ion/rail rifles
and a squad of 2-3 crises suits w/ plasma/fusion.
Fireblade's squad should get at least 2 markerlight hits + cause some damage on target.
Next squad of fire warriors fires at +2 BS, getting hopefully another 2 markerlight hits + some more damage.
Pathfinders shoot next at +2 BS, land 4 markerlight hits and some more damage.
Finally the crises suits shoot at +2 BS and Ignores cover wiping out the remains of the unit.
There you have it, the majority of a large dangerous enemy squad hopefully pulverised in a turn of shooting from less than half of a 1750pt army.
This tactic could be used indefinitely if enough squads have 2-4 markerlights.
Also note that if enough squads roll well, ie. almost every markerlight hits (not gonna happen with my luck
) then more units can shoot with increased BS if necessary, or a better option more markerlights can be used to ignore cover to units that fire before the crises suits.
robo