40 man space is hardly an issue at this point for the Twins, there are SEVERAL players who can be taken off.
In limited MLB action he has a 2.20 ERA or so and 35k in 32 innings and throws 96. I'm sure that is what the Twins like about him, maybe he never comes back from injury or maybe he just never is effective again, doesn't really hurt them and if Rick Anderson can work a little magic you are looking at a possible nice bullpen option who is cheap for several years.
If he recovers well from injury I am sure the Twins will make use of his arm. It just seems like we have relief pitchers with no where to go. Pelfrey will be back next year and possibly in the pen or knocking someone else to the pen so it doesn't fill a need which is disappointing.
Granted I am happy they got something for Morales. I was thinking a lower level prospect that wouldn't need to be on the 40 man as don't you need spots for Meyer, Achter, Sano, Rosario, possibly Buxton, and who knows who else. Seems like those spots fill up quickly.