Tripods are for eliminating all movement (long exposure for light trails mainly), Monopods are to take the weight of a lens. Some find them useful for panning. However i have always found that they cause you to arc your pan (your fixed to one spot on the ground), so i would only really use it for a large long lens such as 400 2.8 (this hasn't been an issue so far).
300 2.8 is hand holdable and anything smaller deffinately doesnt need support unless your a girl.