He's always been the front-runner.
There's one simple reason Alexi Casilla has been the front-runner for that final bench spot: he's out of options. The Twins are certainly giving him every opportunity to earn it with his play this spring, but in 20 at-bats he's collected just two hits and three walks. You should never put too much stock in spring numbers, but .100/.250/.150 can't be what anybody involved is looking for.
Casilla remains the front-runner because the one guy who could have played him out of that roster spot, Matt Tolbert, is struggling even more on both sides of the ball. Tolbert went 0-for-3 today and remains hitless this spring, although he's still just one good day away from equaling Casilla. But he's struggled in the field as well, and with the Twins able to send him to Rochester this season he's putting pressure on nobody.
The only other infielders, or at least the only other players who can play infield as part of their repertoire, are guys without major league experience and at a big disadvantage for making the roster. Luke Hughes (.350/.350/.650) has no real defensive position and doesn't provide the Twins with much upside from a bench role with how this team is stacking up, and Trevor Plouffe is getting filler time (4-for-11 in 9 games with one walk) and isn't a serious consideration at this point.
There is the curious case of Juan Portes. The 24-year old is also without a legit position, but he's had some real playing time this spring and has punished the ball (.381/.409/.857). In spite of how well he's hit, Portes is the yearly example of why you don't put stock in spring numbers--he's a .276/.335/.418 career hitter in the minors, and hasn't even seen triple-A yet.
Is there anyone else who could even be considered? Would you give a passing thought to Jacque Jones by adding him to the 40-man roster?
There aren't a lot of other options outside of Casilla right now. It seems like being out of options might be reason enough to give him the job.