I'd agree that offensively they are quite weak. They are cheap scoring units with a few viable options like a mobile heavy weapon and access to melta weapons. I'd use em in the following way:
1) Distant objective holders. You do exactly what vman said and park a minimum unit with a weapon platform on a rear or far flank objective. They hold the objective and provide heavy weapon support to help soften up distant targets. If they get threatened they tactically retreat and fire the heavy weapon on the move.
Note, that the rules for heavy weapons platforms do not take up space in transports. So a wave serpent held in reserve with star engines can drop this unit onto an objective in the mid to late game if need be.
2) Storm guardian offense. Take storm guardians with melta guns in a serpent. Plow them forward and drop em out to melt tanks and assault the contents. They'll prolly die, but they were cheap.