Board Thread:Speculation House/@comment-14138255-20151025160919/@comment-26397825-20151026044218

Perhaps for the doubles round, RWBY chose Weiss and Yang because they have the best balance of abilities; they are technically the best pair-up of the four of them. And for the doubles, they will instead choose the person who is best at soloing it - which, if I recall correctly, was Ruby's schtick; the whole 'Lone Wolf' thing she had going way back in the begining.

Plus, she has a good weapon for both ranged and melee combat (though the scythe works for her because of her semblance) and is the fastest on the team (again because of her semblance). If she doesn't have to worry about the others, she can just go all out - sort of like what she did against JNPR during the food fight after her entire team was knocked out of the fight. No need to worry about them being caught in the shockwave means she can use her abilities to their fullest extent.