If the beta crowd here is somehow still complaining about Scarabs, then I know that the players after December 4th are gonna go crazy. Let me let you in on a little secret...
Scarabs are actually bad.
I'm in Gold/Plat. The last time I saw the Scarab unit was... actually yesterday, so I can't boast about that. Point is it didn't work and he lost.
Scarabs are only good at defending, and even then they need a considerable amount of time and money to set up. It's been said 1,000,000 times but I'm going to say it again.
1.) Use Militia/Riflemen. A barracks->riflemen start is the best way to start almost any match. This allows you to contest early and scout out a good portion of the map. Scarabs cannot contest space and will lose any fight they engage in.
2.) Use Snipers. Any unit with 2 or more spaces of range render Scarabs useless. Just make sure to defend your own long range units from aircraft.
3.) Bait and switch. That red health Rifleman won't do you any good. Send him to his fiery death and that's a 30 dollar Scarab for a 10 dollar mostly dead unit.
If you want to use Scarabs in your deck...
Don't use them as your main defence. Use them to fortify it. They are supporting units and should be used to cover any holes in your formation. Place them on the left and right sides of the missile pads to stop approaching threats. Use them to guard flank routes to stop Attack Bikes and such. Use them to protect your harvesters.
That is all.