i think to be fair its a mistake to group all these hand together - for example a pair of 5's has far less strength than a pair of nines.
Nines (and sometimes eights), I will raise preflop depending on position and previous action in the hand, and then be looking to make a move with them depending on position and what the flop brings (a scary flop for you is a scary flop for the other players too).
Lower than that, then yeah, limp time, and hope to catch a set. Sevens seem to be unnaturally lucky for some reason