Can't knock this pick. I'm a sucker for any and all offensive line help. The Cards recently re-signed OT Levi Brown, former fifth overall pick who hasn't been elite in his five-year NFL career. Martin gives the Cards more help and helps solidify the offensive line, while also pushing Brown.
SB Nation's Cardinals blog, Revenge of the Birds, explains the pick:
The offensive line is clearly the Cardinals' greatest need. With both David DeCastro and Jonathan Martin on the board, the decision is tough. DeCastro could be an elite guard. Martin has great film, but has one glaring weakness -- his weakness (lack of strength). In the end, Martin is the guy to go with because the Cardinals seek a tackle, especially someone who could play left tackle. Martin's technique is great, his athleticism is without question and if strength is the only thing lacking, he can acquire that with John Lott, one of the best in the business at strength and conditioning.
Jonathan Martin 2012 NFL Draft Profile (via sbnnfl)