If you put the move in perspective, especially acknowledging the Jaguars are in the midst of rebuilding and obviously will not be competing for the playoffs in 2013, it looks like a smart move. While the team sold drafting a "right tackle" with the second overall pick as wanting two premier offensive tackles, it now seems clear Monroe was not in the team's long-term plans.
With just 12 games remaining on his rookie deal, Monroe is a free agent at the end of the season unless the Ravens hand him a new contract, which is very possible. While Monroe is a good left tackle and only 26 years old, the fact that any team trading for him would either have to "rent" him for the remainder of the season or give him a new contract lowers his value.
With the Jaguars not looking to re-sign Monroe or franchise tag him, the best they could hope for was a compensatory pick which maxes out as a third-rounder, which is insanely tough to get. So more likely the team was looking at a fourth-round compensatory pick, which cannot be traded, but instead get multiple third-day picks. Plus, the Jaguars get the picks for the 2014 draft, whereas with compensatory picks they would not be until 2015.