To look at Arsenal's signing of Kim Källstöm, it's probably best to put away one's disappointment about Arsenal's apparent failure to capture a striker (and, for the record: was there a striker available who was better than Olivier Giroud and Nicklas Bendtner? Dimitar Berbatov may be technically better than both, but Fulham were quite willing to be shot of him, which speaks volumes). After Aaron Ramsey's 6-week setback, Arsenal have been left with four midfielders, one of whom, Mathieu Flamini, is banned for four weeks, another of whom, Jack Wilshere, is out and has a long-standing ankle problem, and the third, Alex Oxlade-Chamberlain, has played three games in central midfield for Arsenal. There was, clearly, a short-term need. Arsenal apparently enquired for Blaise Matuidi and Morgan Schneiderlin, but the former is signing a new contract at PSG, and Southampton likely told Arsenal where they could stick their bid. Signing one of those two would've made sense, as they could take over from Arteta or Flamini when Ramsey returned.
However, having not been able to acquire a target that made long-term sense, Arsenal scoured the short-term market, and chose Kim Källström. Källström is 31, and will not wow anyone, but will be a solid depth addition (think of Mikel Arteta, but not quite as good, and with a left foot). For Sweden and for Lyon, his previous club, Källström was not a defensive midfielder, but someone who played well in a double pivot, defending when needed and contributing to the attack, while not putting his side under pressure. He has an excellent range of passing, and can come up with the occasional long-range goal, and by all accounts, he's an incredibly hard-worker. There are some legitimate concerns about his pace, and he's not going to replicate either the direct, goal-scoring runs of Aaron Ramsey, or the intricate play of Ramsey and Jack Wilshere. He will not be, though, very much useless in the final third, unlike Mathieu Flamini.
And, furthermore, Källström probably won't have to play that much: As long as Jack Wilshere can stay injury-free, Arsenal should be able to cover the 6 weeks without Aaron Ramsey. If he does play, it's better than throwing out Gedion Zelalem or Kris Olsson into a high-pressure situation that they are unprepared for, whereas Källström has experience and nous about his play. A lot of Arsenal's sustained title run this season has been down to making calm, good in-game decisions: in Källström, they have another player who won't panic when the chips are down and who will add the technical and tactical quality needed to survive those situations.