It lets you do some pretty weird things since you *are* paying for WS4 and I5 already.
If you tack two more men onto your splinter cannon team, and give the 3 extra jokers CCWs and Pistols you've got a unit that can probably beat four marines in assault, if you shoot them first. 5 Marines if you go Dracon and give him a nifty killy thing.
It doesn't fit the unit very well in most games, admittedly. Usually your fire support is either A) a major friggin target or B) is shooting from as far away as possible.
If your Trueborn happen to have assault weapons, and they happen to be in range of the thing you want them to trash, they do a tad better in shooting + assault than in just shooting. Unlike Kabalite Warriors, who are always strictly better in shooting than in CC, unless the number of assault weapons & CC dudes is greater than the number of dudes with splinter rifles.
Time for the grateful DEldar.