My vote with scarabs is as a cover save wall as your warriors advance, a bodyguard for your destroyer lord- you do have a destroyer lord?, and as late game contesters.
String the scarabs out in cover for the 2+ cover save and string your warriors out behind for the 4+ cover save. If there is no cover for the scarabs, start them a ways back from where you want to go and turbo boost forward for the cover save.
T6 ,3+ save and WBB on the lord is good, but he is going to attract lots of fire and you want to keep him up for last turn contesting and mop up. Attach some scarabs to him and take the wounds on them- he can also turbo-boost and keep up with the scarabs.
Finally, where they really excel is moving as jetbikes which means they can turbo-boost and contest objectives like crazy- especially with the 2+ cover save. It ‘s a shame they are fast attack choices like destroyers since both units are needed in bulk!