For the last three years, the Twins have not added a third catcher as the 14th position player, so that is just trying to justify keeping Jackson on the roster.
It's a choice the Twins make not to use all 13 position players on the roster. One that most teams I believe do not make. Our positional flexibility is based on certain guys can stand at certain positions that weakens the defense, so I am not sure that is being used to our advantage.
There have been a couple of situations just in the last week alone where they could have pinch hit but no one available due to the lack of utilization of Jackson. And there have been other situations, these are the most recent. This roster construction is flawed and right now is one of the factors of a sub .500 team. At some point you can't keep beating your head at the wall trying the same thing over and over and expecting different results.
As for calling up Jenkins, sometimes you have to take a risk to help your team and this might be one of those times. Zoll needs to walk across the street and talk with Tim Connelly about building a winning team and being willing to take risks to do so.