I used my Master Ball on Volcarona, because I caught all of the other Legendaries by other means, to challenge myself, and I don't want to deal with a level 70 Bug/Grass type, considering my starter is Serperior. XD So, yeah, I got lazy and went for it. The first time I played through SoulSilver, I didn't use my Master Ball at all. I caught Lugia, Ho-oh, Mewtwo, Latias, Groudon, the three birds, AND the three beasts all with Ultra Balls, Dusk Balls, and other stuff. So, I just had a Master Ball wasting space in my bag. :P