pistoliers are excellent as a heavy cav/infantry killers, put them on a flank and fire your 12 shots (8 on 5+, 4 on 4+) and just watch. I've managed to wipe out 5 Blood Knights: in shooting phase i took down 3 of them, the other 2 in a stand&shoot reaction. That paid off well(and they did have flag of the blood keep).
They also do well against war machines and as a chasers. Can help to bring down some monsters too.
I don't use them as a redirectors, it is just not pointwise. They are specialist unit and treat them like that. Theirs most essential asset is to march and fire hard hitting missiles. They can outmanouvre(i'm not sure if i spelled that word correctly

) most of h/cav units and give them a solid punch in the side or back.