Nothing wrong with drafting RB in the second. After all, Thurman Thomas was a second round pick, 40th overall I believe.
Other recent second or later round notables:
2006: Maurice Jones Drew (2)
2005: Frank Gore (3)
2004: Tatum Bell and Julius Jones (2), plus Michael Turner in the 5th
2003: Domonick Davis (4)
2002: Dashaun Foster, Clinton Portis, Ladell Betts (2) and Brian Westbrook (3), CHESTER TAYLOR (6th round)
2001: Travis Henry (2), Rudi Johnson (4)
That's just in the last 6 years. In that same time there have been at least 5 first round flops.
