Turbo boosting scarabs are impossible to kill with the 2+ cover save and being fearless so don’t have any worries about that- as long as you are not assaulted you’ll be about with enough of them to contest at the end of turn 5/6.
Depending on what is going on with the mission, and the list I’m running- especially my foot lord + immortal list there are times I don’t want X of my opponent’s unit to shoot at my immortals. Sure I have solar pulse for this reason (100% needed with Necrons, always, period!) but what if it is needed later or used up because of Dawn of War?
Bring in the scarabs!
Keeping my scarabs center in the table means I can pretty much reach anything in a turn of turbo boosting. When my opponent moves up a unit that I know is going to unload on the immortals next turn, I’ll boost my scarabs over to said unit making it look like I’m going to attack it in the assault. Almost every time my opponent shoots at said scarabs, and making a 2+ cover save is easy. Next turn the immortals escape and the scarabs move away….