scarabs fit nicly into most necron armies, but if you plan on using a lot then they have to be for a specific purpose, a lord with lighning fied, gaze of flame and destroyer body joined with them makes them excellent assault defence. As a counter assault, put them behind the warriors and spring them on any enemy that get within 18". They usually hate hits but hold up the enemy for so long, wyches, banshees, incubi, honour guard, stealers, the lot