They want to win games. They are not doing this to take a look at Johnson really. In a development world he'd have a couple more low key starts and go home for the offseason.
Johnson is the best alternative of their current choices at Lehigh Valley because Abel has disappointed and everyone else is mediocre. Allard is a little better than Phillips so could be in play after his 15 days in the minors are up, but it is not as if he has looked that great so far this year.