The lefties are a little less valuable with all left-handed starters, but longer games and having redundancy is the way they have operated in the past.
Also, the right-handed options are just not that great. I suppose all of Nola, Walker, and Buehler could be on the roster, but that seems overkill if a 4th starter is not needed. At the moment I guess 2 of 3 with Walker being the likely odd man out. Buehler has some decent relief experience in the playoffs and Nola makes it out of respect. And I guess a tandem start in some way with Luzardo or Suarez is possible. Just because Nola has never relieved does not mean he can't - and Walker just is not that good.
Nola's K/BB ratio is still decent and that is kind of what you want in relief rather than Walker pitching to contact.