Uses
Single battle
You can use it to eliminate opponents abilities that would be potentially dangerous to you (like Gastro acid). Example: You are fighting Sand-rush Excadrill, you can use worry seed to remove its speed boost.
Double/Triple battle
You can use it to remove a hindering ability from a teammate. Slaking with Truant, Regigigas with Slow-start, Archeops with Defeatist etc.