It would seem they do that to prevent the meta and/or fotm picks from getting 99% ban rates or outright perma-ban. And instead of letting the ban rate give them a call to force them into dealing with the characters who might get insanely banned for whatever reason, via reworks, changes or nerfing, they simply opted to change the system design in such a way that the worst of the worst don't appear so bad even with 10-bans.
I don't think that's the correct attitude really.
Sure, Riot may have other reasons and considerations. But much like that, there are also other slightly alternative realisations or adjustments from the 10-ban general idea extended from our good o' 6-ban that could all do those jobs without resorting to this "blind" draft that more or less defeats the point of drafting in the first place.
But what do I know, I'm no statistician nor am I a pro MLG Challenger, this is my personal opinion on what I have gathered so far, it's subjected to be lacking information and/or misinformed.