Many fans speculated the move was made as a result of Marc Staal being injured. That turned out to be untrue, as multiple reports pointed towards the Rangers discussing bringing in Hamrlik before the Rangers beat the Philadelphia Flyers (and thus before Staal was injured).
The move, when you take a step back and look at it from an expectations perspective, makes a ton of sense. Glen Sather obviously believed the Rangers needed some defensive depth. The Staal injury proved just how important that defensive depth in. Remember, teams overpay for guys like Hamrlik at the trading deadline every year, the Rangers picked him up for nothing more than the monetary factors that come into play.
The Good: For Hamrlik, 2011-12 was essentially "A Tale of Two Coaches," as he struggled under Bruce Boudreau and turned things around 180 degrees under Dale Hunter (who, curiously, benched the veteran blueliner for... wait for it... taking bad penalties). To be sure, Hamrlik was a victim of some bad puck luck under Boudreau, but his play was the primary contributor in a woeful goal differential (he was on ice for 15 Caps goals and 31 against in 22 games played) that nearly mirrored his disparity under Hunter (42 Caps goals, 24 opposition tallies in 46 games). At five-aside, Hamrlik got tough zone starts, but faced relatively weak competition and posted the best relative Corsi among the team's blueliners, and on the penalty kill he performed similarly - hard to ask for much more from the defensive conscience of the second D-pairing.
The bad portion of Hamrlik's review has to do with the lack of his offensive abilities when compared to what he did earlier in the year.
But the reality of the situation is Hamrlik's offensive output is moot right now. The Rangers aren't bringing him back to be a solution on the power play or to add offense. Hamrlik is here to be a bottom pair defenseman who is a true upgrade over Stu Bickel.
The only real downside is that the Rangers will lose some mobility at the trading deadline, but that's not as big of a deal for me. A ton of teams are going to overpay at the deadline, especially this year. The Rangers just added a veteran presence on their blue line. If they get Matts Zuccarello then they've also bolstered their offense and their power play for nothing more than money.
At the end of the day, if you take the move for what it's worth, it's a good move. If you have expectations that Hamrlik is supposed to be on the top pairing and is going to solve the power play, well, you're probably going to be disappointed.