Texans | R. Bush not a lock to be No. 1 draft pick?

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I hear alot of people say take the best possible player available in the draft. Lets say we had the number one draft spot would we draft Reggie. He would cost alot of $ and we already have Portis and El who are proven players in the NFL. I just don't understand the concept of taken the best player even though there is not a need at the time and wasting a pick and the $ on an unproven player when there is not a need.