Zeller and Green signings are both puzzling from a roster standpoint. Signing them makes an already over flowing roster worse.
In terms of Zeller, I don't see why they would sign him just to be trade ballast. They can't trade him soon enough that someone would need to be cut. I think more likely, they intend to trade someone else, perhaps Olynyk or Amir or both. This would fit the narrative of the now very stale rumors about Griffin or even Cousins. I know both of these are very unlikely but it seems that something could be in the works where we send out more players than we get back, in particular bigs, after which we need Zeller on the Bench.
It really just depends how much you think the roster is truly overflowing.
Excluding Green and Zeller, look at the last few players on the roster, in Young, Hunter, Mickey, Bentil, and Holland. None of them have guaranteed contracts next season. Young and Hunter both have options that need to be exercised by late October. Mickey isn't guaranteed, Bentil isn't guaranteed, and Holland will be a free agent. None of them have shown much of anything outside the D-league that creates any argument they should on the roster long-term. Accordingly, you could look at all 5 as young guys on expiring contracts. So if you think Gerald Green is better this year than James Young and RJ Hunter, then why not sign him to a one-year deal? If both of those guys completely outplay Green in camp, then you can still move on from Green. If neither do, and neither show any improvement, then you can move on now, while still having a viable bench player. If one does, then you can still keep him. The same could be true of Mickey and Bentil vs. Zeller, although Zeller will not be released during camp. But if neither are capable of giving you NBA minutes, it's good to have someone on the bench who can.
Based on what we've seen to date, it seems unlikely that more than two of Hunter, Young, Mickey, Bentil, and Holland should be seen as potential options for the 2017-2018 roster, much less this season. You can't use 4-5 roster spots on D-league projects currently. Injuries are too frequent, and you can only send two down to the D-league at a time. It's time the wheat separates itself from the chaff.