Bringing him in accomplished so much more than just getting an extra solid inning. ("Solid" being a huge understatement.) It re-established his confidence, which must have taken a big hit with his game 2 performance. By letting him pitch to live batters on his throwing day it gave him a much better tune up than just throwing a bullpen session. He'll be sharper his next start (Monday) for it.
You know in the "old days", managers would routinely put a starter in for an inning between starts on their throwing day, for just that reason, plus the fact that it relieved pressure on the bullpen. I don't know why they no longer do it.