They don't need four starters in the first round, but they do need to keep a starter (presumably Falter) off the roster and ready to start a possible Game 1. Thor seems like a better fit than Gibson out of the pen but we can't rule out Gibson being back in the rotation if they get to the NLDS. I'd still be equally nervous counting on Falter, but the previously inconsistent Gibson seems to have fallen off the cliff completely.
And if it gets to a Game 3 and Suarez isn't perfect he's not going to get to pitch out of trouble in the third or fourth inning. They might even script for someone replacing Suarez before the third time through, if not an opener (depending on the opponent's line-up).
I still have this feeling that Thomson would rather have Munoz or Maton (assuming Sosa is back) or Guthrie as his fifth bench guy than Hall. But - there's no ghost runner in the playoffs either right? So that extra pure PH, and not having to worry about PRing for possible ghost runners, also a consideration. They could also go with Hall in the first series and worry about having a more versatile guy if there's a second.