A-the unit to be charged is severely weakened from enemy fire.
B-the unit is weaker than you.
C-it's to avoid a charge from the enemy, and deny them a charge bonus.
The main bonus of Thousand Sons, since 5th edition, has always been their AP3 bolters. They still get them, and with the average cover save going down to 5+, they are more likely to kill. The reduced cover makes you own 4+ invulnerable save more worthwhile. Another boost you can take is the Icon of flame for 15 points. It's your only option, but it can help against weaker horde units, as it grands the squad soul blaze. Not a particularly powerful ability, but trust me, it does help in thinning down units. Managed to take down a Meganob with it once, so it with good luck it can help in all situations.
So, with these tips, you can now get the most from your blue bodiless automatons of Chaos.