Stipulating that of course you want to give Sullinger and Olynyk a lot of minutes, that doesn't mean they will soak up 100% of the minutes at power forward. There should be a non-zero number of minutes where the Celtics will have to play either Bass or Humphries, assuming both are still on the roster during the season.
I'd prefer to play Bass because Humphries' trade value is more dependent on him being an expiring contract than as player teams want. Additionally, I can see scenarios where Bass is on the team beyond next season (for example, if one young PF is used as trade fodder to acquire a top center, Bass as a veteran backup would make sense).