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A lot of things in this thread are wrong. Yes, sivir is good at pushing, but it is not why you take her. Pushing side lanes by yourself is not a very rare or valuable skill. Clearing creep quickly is not rare or particularly valuable either.
The reason you take sivir is twofold:
Because of her ult. Global movespeed and attack speed means a hell of a lot in a team fight.
Because she has total dps higher than anyone else in the game with the exception of twitch with his ult up, and everyone standing in a line. A sivir with a couple damage items and some team synergy (warwick, nunu, whatever) can melt an entire enemy team in 5 seconds flat.