First, no, Davis didn't play better than Posey this year. Davis played above what we expected, but it wasn't good enough because he can't defend. Maybe Posey isn't an elite perimeter defender at this stage, but he is ridiculously superior to Davis in that category. If we had Posey instead of Davis on our team this year, we would have beaten Orlando. For all Davis added in terms of offense, remember Orlando didn't respect his shot and often left him open to double on Ray and Paul. The last second shot was an extreme example, but a good indication of how Orlando's defense prioritized keeping a man on Baby. With Posey, they stay on him and respect his shot and it opens up more opportunities for other players.
Now, I think the OP does understate Davis's contributions. It wasn't that negative. It wasn't as positive as some people have said (he doesn't have a good jumper, he's a bad defensive rebounder and all around defender), but it wasn't negative (good offensive rebounding, okay shot, decent court vision and handle for his size, intensity and intangibles). He's also young and has shown improvement. I still think we should keep Davis at anything up to $4 mil a year and maybe consider matching above that too.
While I agree that he's not needed for that many minutes barring injury, and there will be situations where those minutes would be better filled by Scal, the way I look at it is that Davis is an asset. He has some value. He's young, he's big and he can play in the NBA. That has inherent value. We don't have many ways to get a piece that we need now or may need during the season. We have $2 million in the biannual exception and as many minimum contracts as we want to give out/have roster space for. That isn't a lot to offer, especially as free agency dries up. Our best bet to add a good piece (i.e., not a stopgap like Mikki Moore) now or later is through a trade. Davis is at the very least an asset toward such a trade. If we let him walk to pick up a minimum salary guy, we don't just risk the player being inferior, we also risk not having the necessary pieces to make a trade to help us out.
Now, clearly BYC rules will make it somewhat difficult to trade Davis, a situation which would be even more difficult if he signs an offer sheet and we match, imposing a lot of restrictions on trading Davis. However, if we can wrap him up before that happens, we still at least hold onto an asset.
In the present six to eight team arms race going on in the NBA, I just think it's a bad idea to let assets go. That's why the Orlando move in matching the Gortat offer, expensive as it was, was such a big addition for the Magic - they are not giving away assets. They're holding onto them. As expensive as it is, in this heightened competition, that's what we have to think about. We've got 7 solid rotation players (starters, Wallace and House), a couple last resort injury fillins (Scal, maybe TA), some young projects with limited trade value (Walker, Giddens, Pruitt) and limited trade assets (future firsts, but I think 2013 is the first one we could give; bad second round picks; Scal and TA's expiring deals; low value young projects). Davis gives you at the very least another injury fill in, a trade asset, and maybe even another solid rotation player if he makes strides on last year. I just think with our limited additional assets, we need to hold onto a guy like Davis. And to protect tradeability, I'd love to see us offer him a 2 year $6 million deal to see if he'll take it or be actively exploring sign and trade deals now.