Why did the Bengals draft a RB in the second round?  I think two recent developments helped move that position up the need board.  One was DT Michael Myers signing for two years.  That signing pretty much eliminated the need to draft a DT in the first or second round.  Lewis much prefers to draft DTs on the second day, and the Myers signing let him go that route, It gives Matt Toeaina a year or two to develop his skills.

The other factor was Chris Henry's suspension.  Henry is perhaps most important to the offense in the red zone; he might even be Carson Palmer's favorite goal line target.  With him gone, the red zone passing offense simply won't be as good.  That puts additional pressure on the run offense to be more productive.  My guess is that the coaches looked critically at what they had in Rudi Johnson, Kenny Watson and a bunch of who-dats and decided they needed to upgrade the red zone running game.  In particular, Henry's suspension took away the option of standing pat, going with last year's lineup of Rudi Johnson and Kenny Watson, and giving Chris Perry as much time as he needed to get healthy.  

Irons was drafted to make the red zone running offense more of a scoring threat.  That will continue to have dividends when Henry finally comes back.