If I have followed along correctly, we have signed Kornet (Roster spot 11) and Hauser (spot 12) plus we have signed Davison to one of the 2-way spots. That leaves 3 roster spots and 1 2-way spot. I suspect that Jackson is not eligible for a 2-way spot (but not sure).
For the 3 regular roster spots, one of them will almost certainly go to a vet big. That leaves 2 spots. There has been a lot of debate as to whether we should fill these with rotation ready vets or with more upside types of players. I am not sure that Jackson is either but he certainly seems to be capable of being a fill in for some regular season minutes when we have injuries or foul trouble. He does not seem like some one who could be a longer term replacement if we have a prolonged injury to a rotation player. And there probably is not a lot of upside left for him.
I am actually rooting more for Brodric Thomas. He may end up on another 2-way though as I believe he would still be eligible, which is fine for the Celtics). Thomas is really smooth. He looks like an NBA player to me. He can be that fill in guy now and I think there is upside with him. He has a chance in my opinion to become a legit NBA bench player.
I like Thomas too but Jackson fills more of a need for a backup 3-4.
Justin Jackson as a "4"? I guess but he would be behind Horford, Gallinari, and Grant. Not sure I agree there is more of a need there as compared to just a regular wing. Wings can be more of a guard'ish wing or a forward'ish wing. Jackson is more of that forward'ish wing, I agree. That could allow him to defend some bigger wings over what Thomas can but I don't see Jackson being in the mix at PF. Maybe that is what you mean.
If Thomas is potentially better now and with more upside, I still keep him over Jackson but it is a tough call. If Jackson is capable enough to be a reliable bench wing now, then great. Is he better than Nesmith for example? Probably not so he would play less than Nesmith. If that is the case, I prefer to continue to look at Thomas.
There is very little that is going to separate these players being considered for spots 14 and 15. I would have no issue if Jackson makes it.