Since Murderous Imp does 1 direct damage to any ally of your choice upon entering play, you can get rid of your opponent's ally at the same time as playing your own, giving you a unique edge (two birds with one stone!). If you have a Murderous Imp but your opponent hasn't played anything yet, bench your Imp and wait-- doing 1 damage to a low-HP ally will have a lot more impact than 1 damage to your opponent.
Strong against: low-HP allies and fast decks (Murderous Imp is a good defender!).
Weak against: quality allies, slow-developing decks.