Here's another couple uses for pistoliers that haven't been mentioned. Take an infantry unit that scares your squishy state troopers. Let's call it, hypothetically, "khorne halberd warriors" or "temple guard with slann" or "bloodletters" or even "black guard."
Step 1: Place your pistoliers in front of it, one inch further away than their base movement.
Step 2: Shoot them. You might kill one or two. Meh.
Your opponent now has a choice. He can either charge them, and he has to roll higher on 2 dice than you roll on swiftstride. This is exceedingly rare. In effect, you have just reduced the movement of this very scary unit to d6 per turn. This gives your guns (you DID bring some guns, right?) a couple of extra turns to shoot them, because the thing is, you can rally and do this again every single phase.
This will work best when facing an army like lizardmen or chaos that lacks any cavalry that can chase you down.
Also worth noting is that pistoliers can go and stand on a mangla squig. Sometimes it's worth it.
They're not useless, but they're limited enough that they rarely make it in my lists under 2500. Above that point total, I rarely leave them out.